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'''電塔'''，又名'''輸電塔'''或'''輸電鐵塔'''，是用來承托[[架空電纜]]的[[結構|結構物]]，通常為鋼製{{tsl|en|lattice tower|鐵塔-}}。[[輸電網路]]中的[[輸電系統]]主要用於大規模從[[發電廠]]輸送電力至負載中心，使用架空電纜相對地底電纜成本較低，故需要輸電塔將電纜抬高以避免高壓電力影響地面活動。較低電壓的[[配電系統]]的則常用[[电线杆]]作支撐物。電塔有各種不同形狀和大小，高度通常為15至55米之間，但最高可見於{{tsl|en|Zhoushan Island Overhead Powerline Tie|舟山島架空電纜}}，當中有兩座370米高的輸電塔。除鋼鐵以外，亦有見以[[混凝土]]或木材作為建築材料。
{{NoteTA|G1=物理學}}
{{infobox| title = 光速
| image = -{zh-hant:[[File:Sun to Earth-zh-hk.jpg|300px]];zh-hans:[[File:Sun to Earth-zh-hans.jpg|300px]]}-
| caption = [[太陽光]]平均只要8分19秒即可到達[[地球]]。
| header1 = 準確數字
|labelstyle = font-weight:normal
| label2 = [[米每秒]]
| data2 = {{val|299792458}}
| label3 = [[普朗克單位制|普朗克]]
| data3 = 1
| header4 = 大約數字 <!--3 s.f.-->
| label5 = [[公里每小時|公里每秒]]
| data5 = 300,000
| label6 = [[公里每小時]]
| data6 = 1,080,000,000
| label7 = [[英里每小時|英里每秒]]
| data7 = 186,000
| label8 = [[英里每小時]]
| data8 = 671,000,000
| label9 = [[天文單位]]每日
| data9 = 173
| header10 = 前進某一距離所需時間
| label11 = '''距離'''
| data11 = '''時間'''
| label12 = 1[[英尺]]
| data12 = 1.0[[納秒]]
| label13 = 1[[米]]
| data13 = 3.3納秒
| label16 = 從[[地球靜止軌道]]到地面
| data16 = 119[[毫秒]]
| label17 = [[赤道]]長度
| data17 = 134毫秒
| label18 = 從[[月球]]到地球
| data18 = 1.3[[秒]]
| label19 = 從[[太陽]]到地球（1[[天文單位]]）
| data19 = 8.3[[分鐘]]
| label21 = 從[[毗鄰星]]到太陽（1.3[[秒差距]]）
| data21 = 4.2[[年]]
| label23 = 從[[大犬座矮星系]]到地球
| data23 = 25,000年
| label24 = 橫越[[銀河系]]
| data24 = 100,000年
| label25 = 從[[仙女座星系]]到地球
| data25 = 2,500,000年
}}
'''光速'''，通常指[[光波]]傳播的速度<ref>{{cite book|title=现代汉语词典|edition=第五版|year=2005|publisher=[[商务印书馆]]|isbn=9787100043854}}</ref>。光在真空中傳播的速度，又名{{Smallmath|f=c}}，是一個於[[物理學]]中極為重要的[[物理常數]]。此值為299,792,458[[米每秒]]。其為一實數，因為[[秒]]為國際單位，而[[米]]的長度亦由光速定義<ref name="penrose">{{Cite book|last=Penrose|first=R|year=2004|title=The Road to Reality: A Complete Guide to the Laws of the Universe|pages=410–1|publisher=[[Vintage Books]]|isbn=978-0-679-77631-4|quote=...the most accurate standard for the metre is conveniently ''defined'' so that there are exactly 299,792,458 of them to the distance travelled by light in a standard second, giving a value for the metre that very accurately matches the now inadequately precise standard metre rule in Paris.}}</ref>。以英制單位來說，此值約為186,282尺每秒。


電塔可主要分為三大類：{{tsl|en|suspension tower|懸吊塔}}、{{tsl|en|Dead-end tower|張力塔}}以及{{tsl|en|transposition tower|轉置塔}}。有些電塔則同時有以上數項塔種的功能。電塔和架空電纜為一種{{tsl|en|visual pollution|視覺污染}}，故亦為{{tsl|en|undergrounding|管線地下化}}的其中一種理由。
跟據[[狹義相對論]]，{{Smallmath|f=c}}是宇宙中所有能量、物質或[[資訊 (物理學)|資訊]]所能達至的最高速度。{{Smallmath|f=c}}也是[[無質量粒子]]<!--massless particle,無誤？-->及相關的[[場 (物理)|場]]（包括[[電磁波]]，如[[光]]）於真空中前進的速度。其亦是現時理論中預測[[重力波 (相對論)|重力波]]的[[重力速度|傳遞速度]]。上述的粒子或波皆以{{Smallmath|f=c}}傳遞，不論來源有否[[運動 (物理學)|運動]]或觀察者的[[慣性參考系]]。[[相對論]]中，{{Smallmath|f=c}}與[[時空]]相關連，亦出現於[[質能等價]]公式{{Smallmath|f=E=mc^2}}之中<ref name=LeClerq>{{Cite book|last=Uzan|first=J-P|last2=Leclercq|first2=B|year=2008|title=The Natural Laws of the Universe:Understanding Fundamental Constants| url=http://books.google.com/?id=dSAWX8TNpScC&pg=PA43 | pages=43–4 | publisher=Springer (publisher)|Springer|isbn=0-387-73454-6}}</ref>。


== 結構 ==
光線傳播時若通過[[透明]]的物質，如空氣或水，則其速度會低於{{Smallmath|f=c}}，而該速度{{Smallmath|f=v}}與{{Smallmath|f=c}}的比例為[[折射率]]－{{Smallmath|f=n}}（{{Smallmath|f=n=\tfrac{c}{v} }}）。例如[[可見光]]於[[玻璃]]的折射率通常約為1.5，即光於玻璃中傳播時的速度為{{nowrap|{{Smallmath|f=\tfrac{c}{1.5} \thickapprox 200,000}} }}公里每秒；空氣的折射率約為1.0003，即光於空氣中的速度比{{Smallmath|f=c}}慢約90公里每秒<!--TODO:REF-->。
電塔結構的建設費用通常佔該條輸電線路的三成至四成。其設計會因應地貌、氣候，以及架空電纜的電壓、線路數等參數而有所不同。跨臂
=== 種類 ===
=== 力學計算 ===
==== 垂直負載 ====
==== 縱向負載 ====
==== 橫向負載 ====


==== 線段跨度 ====
大部分情況下，光都可以被理解為「瞬間到達」<!--我知道應用主語，但想不出-->，但當距離較長或要求非常精準時光的速度就顯得非常重要。當與遙遠的[[太空探測器]]溝通時，其需要數分鐘甚至以小時計<!--TODO:CLEANUP-->來傳遞訊息。由於星系之間的距離極長，所以我們看到的星光實際上由恆星於很多年前發出，使我們能借此研究宇宙的歷史<!--TODO:CLEANUP-->。光線有限的速度也限制了[[電腦]]於理論上的最高速度，因為電腦中的信息需於[[集成電路]]之間傳遞。最後，光速亦能用於精確測量長距離的飛行。
=== 鋼構連接 ===
=== 特殊設計 ===
Sometimes (in particular on steel lattice towers for the highest voltage levels) transmitting plants are installed, and antennas mounted on the top above or below the overhead [[架空電纜|ground wire]]. Usually these installations are for mobile phone services or the operating radio of the power supply firm, but occasionally also for other radio services, like directional radio. Thus transmitting antennas for low-power FM radio and television transmitters were already installed on pylons. On the [[Elbe Crossing 1]] tower, there is a radar facility belonging to the [[汉堡]] water and navigation office.


For crossing broad valleys, a large distance between the conductors must be maintained to avoid short-circuits caused by conductor cables colliding during storms. To achieve this, sometimes a separate mast or tower is used for each conductor. For crossing wide rivers and straits with flat coastlines, very tall towers must be built due to the necessity of a large height clearance for navigation. Such towers and the conductors they carry must be equipped with flight safety lamps and reflectors.
[[奧勒·羅默]]於1676年首次以[[木衛一]]與[[木星]]之間的掩食演示了光的速度有限。1865年，[[詹姆斯·克拉克·馬克士威]]提出光是一種電磁波，因此在他的理論中為光速予以{{Smallmath|f=c}}<ref>{{cite web|title=How is the speed of light measured?|url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html}}</ref>。1905年，[[阿爾伯特·愛因斯坦]]假設光速對於任何慣性系來說都是獨立於其光源<ref name="stachel">{{cite book |title=Einstein from "B" to "Z" – Volume 9 of Einstein studies |first1=JJ |last1=Stachel |publisher=Springer |year=2002 |isbn=0-8176-4143-2 |page=226 |url=http://books.google.com/books?id=OAsQ_hFjhrAC&pg=PA226}}</ref>，並根據[[狹義相對論]]探討了一些推論，並由此顯示{{Smallmath|f=c}}不只在光和電磁方面的相關性。經過百多年來越來越精確的測量，1975年測出為約為{{val|299792458}}米每秒。1983年，[[國際單位制]]將[[米]]按光速重新定義，改為每秒的299,792,458分之1<ref name="SIbrochure">{{SIbrochure}}</ref>{{rp|112}}。


Two well-known wide river crossings are the [[Elbe Crossing 1]] and [[Elbe Crossing 2]]. The latter has the tallest overhead line masts in Europe, at {{convert|227|m|ft|abbr=on}} tall. In Spain, the {{tsl|en|overhead line crossing|}} pylons in the Spanish {{tsl|en|Pylons of Cadiz||bay of Cádiz}} have a particularly interesting construction. The main crossing towers are {{convert|158|m|ft|abbr=on}} tall with one crossarm atop a [[锥台]] framework construction. The longest overhead line spans are the crossing of the Norwegian Sognefjord ({{convert|4597|m|ft|abbr=on}} between two masts) and the {{tsl|en|Ameralik Span|}} in Greenland ({{convert|5376|m|ft|abbr=on}}). In Germany, the overhead line of the EnBW AG crossing of the Eyachtal has the longest span in the country at {{convert|1444|m|ft|abbr=on}}.
{{TOC limit}}


In order to drop overhead lines into steep, deep valleys, inclined towers are occasionally used. These are utilized at the [[胡佛水壩]], located in the United States, to descend the cliff walls of the {{tsl|en|Black Canyon of the Colorado|}}. In Switzerland, a pylon inclined around 20 degrees to the vertical is located near [[薩甘斯]], [[聖加侖州|St. Gallens]]. Highly sloping masts are used on two 380 kV pylons in Switzerland, the top 32 meters of one of them being bent by 18 degrees to the vertical.
==數值、符號和單位==
光在真空中傳播的速度通常以{{Smallmath|f=c}}代表，而{{Smallmath|f=c}}則意為{{lang|en|constant}}（常數）或拉丁文{{lang|la|celeritas}}（迅捷）。一開始使用的符號為[[詹姆斯·克拉克·馬克士威]]於1865年發明的{{Smallmath|f=V}}。而{{Smallmath|f=c}}原本，由1856開始，[[魯道夫·科爾勞施]]與[[威廉·韋伯]]皆用之於真空中光速的{{Smallmath|f=\sqrt{2} }}倍。1894年，[[保羅·德汝德]]將其重新定義至現有意思。[[阿爾伯特·愛因斯坦|愛因斯坦]]於1905年的[[奇蹟年論文]]使用{{Smallmath|f=V}}，到1907時則轉用{{Smallmath|f=c}}，其後{{Smallmath|f=c}}更成為了一標準符號<ref name=Yc>{{cite web|last=Gibbs|first=P|year=2004|origyear=1997|title=Why is ''c'' the symbol for the speed of light?|url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html|work=Usenet Physics FAQ|publisher=University of California, Riverside|accessdate=2009-11-16|archiveurl=http://www.webcitation.org/5lLMPPN4L|archivedate=2009-11-17}}</ref><ref>{{cite journal|last=Mendelson|first=KS|year=2006|title=The story of ''c''|journal=American Journal of Physics|volume=74|issue=11|pages=995–997|doi=10.1119/1.2238887|bibcode = 2006AmJPh..74..995M }}</ref>。


Power station chimneys are sometimes equipped with crossbars for fixing conductors of the outgoing lines. Because of possible problems with corrosion by flue gases, such constructions are very rare.
有時{{Smallmath|f=c}}用作光於任何材質中的速度，而{{Smallmath|f=c}}<sub>{{Smallmath|f=_0}}</sub>則用於真空中的光速<ref name=handbook>
*{{Cite book|last=Lide|first=DR|year=2004|title=CRC Handbook of Chemistry and Physics|url=http://books.google.com/?id=WDll8hA006AC&pg=PT76&dq=speed+of+light+%22c0+OR+%22|pages=2–9|publisher=CRC Press|isbn=0-8493-0485-7}}
*{{Cite book|last=Harris|first=JW|coauthors=''et al.''|year=2002|title=Handbook of Physics|url=http://books.google.com/?id=c60mCxGRMR8C&pg=PA499&dq=speed+of+light+%22c0+OR+%22+date:2000-2009|page=499|publisher=Springer|isbn=0-387-95269-1
}}
*{{Cite book|last=Whitaker|first=JC|year=2005|title=The Electronics Handbook|url=http://books.google.com/?id=FdSQSAC3_EwC&pg=PA235&dq=speed+of+light+c0+handbook|page=235|publisher=CRC Press|isbn=0-8493-1889-0
}}
*{{Cite book|last=Cohen|first=ER|coauthors=''et al.''|year=2007|title=Quantities, Units and Symbols in Physical Chemistry |url=http://books.google.com/?id=TElmhULQoeIC&pg=PA143&dq=speed+of+light+c0+handbook|page=184|edition=3rd|publisher=[[Royal Society of Chemistry]]|isbn=0-85404-433-7}}</ref>。此法受國際單位制官方文章認可<ref name="SIbrochure"/>{{rp|112}}，而同時亦存在於相關的常數，如{{Smallmath|f=\mu}}<sub>{{Smallmath|f=0}}</sub>為[[真空磁導率]]、{{Smallmath|f=\epsilon}}<sub>{{Smallmath|f=0}}</sub>為[[真空電容率]]、{{Smallmath|f=\Zeta}}<sub>{{Smallmath|f=0}}</sub>為[[自由空間阻抗]]。此條目使用{{Smallmath|f=c}}代表真空中的光速。


A new type of pylon, called Wintrack pylons, will be used in the Netherlands starting in 2010. The pylons were designed as a minimalist structure by Dutch architects Zwarts and Jansma. The use of physical laws for the design made a reduction of the magnetic field possible. Also, the visual impact on the surrounding landscape is reduced.<ref name=nethertowers>{{cite web|url=http://www.zwarts.jansma.nl/artefact-2410-en.html|title=New High Voltage Pylons for the Netherlands|accessdate=2010-04-24|year=2009}}</ref>
[[國際單位制]]中，米定義為{{Smallmath|f=c}}的{{val|299792458}}分之1秒，因此{{Smallmath|f=c}}也倒過來固定於{{val|299792458}}米每秒<ref name=Boyes>{{Cite book|last=Sydenham|first=PH|year=2003|chapter=Measurement of length|chapterurl=http://books.google.com/books?id=sarHIbCVOUAC&pg=PA56|editor=Boyes, W|title=Instrumentation Reference Book|edition=3rd|page=56|publisher=Butterworth–Heinemann|isbn=0-7506-7123-8|quote=... if the speed of light is defined as a fixed number then, in principle, the time standard will serve as the length standard ...}}</ref><ref name="Fundamental Physical Constants">{{cite web|title=CODATA value: Speed of Light in Vacuum|url=http://physics.nist.gov/cgi-bin/cuu/Value?c|work=The NIST reference on Constants, Units, and Uncertainty
|publisher=National Institute of Standards and Technology|accessdate=2009-08-21}}</ref><ref name=Jespersen>{{Cite book |last=Jespersen|first=J|last2=Fitz-Randolph|first2=J|last3=Robb|first3=J|year=1999|title=From Sundials to Atomic Clocks: Understanding Time and Frequency|url=http://books.google.com/?id=Z7chuo4ebUAC&pg=PA280|page=280|edition=Reprint of National Bureau of Standards 1977, 2nd|publisher=Courier Dover|isbn=0-486-40913-9}}</ref>。{{Smallmath|f=c}}的數值於不同系統也有不同數值，如[[英制單位|英制]]及[[美制單位]]中，若按每寸等於2.54厘米則{{Smallmath|f=c}}為186,282英里，698碼，2呎及{{frac|5|21|127}}吋每秒<ref>
{{cite web|last=Savard|first=J|title=From Gold Coins to Cadmium Light|url=http://www.quadibloc.com/other/cnv03.htm|work=[http://www.quadibloc.com/ John Savard's Home Page]|accessdate=2009-11-14|archiveurl=http://www.webcitation.org/5lHYVsp5E |archivedate=2009-11-14}}</ref>。[[自然單位制]]中，{{Smallmath|f=c}}[[歸一化]]成為{{nowrap|{{Smallmath|f=c=1}}}}<ref name=Lawrie>{{Cite book|last=Lawrie|first=ID|year=2002|chapter=Appendix C: Natural units|chapterurl=http://books.google.com/books?id=9HZStxmfi3UC&pg=PA540|title=A Unified Grand Tour of Theoretical Physics|edition=2nd|publisher=CRC Press |isbn=0-7503-0604-1}}</ref>{{rp|540}}<ref name=Hsu>{{Cite book|last=Hsu|first=L|year=2006|chapter=Appendix A: Systems of units and the development of relativity theories|chapterurl=http://books.google.com/books?id=amLqckyrvUwC&pg=PA428|title=A Broader View of Relativity: General Implications of Lorentz and Poincaré Invariance|edition=2nd|publisher=World Scientific|isbn=981-256-651-1}}</ref>{{rp|427-8}}。


Two clown-shaped pylons appear in Hungary, on both sides of the [[M5公路 (匈牙利)|M5 motorway]], near [[乌伊豪尔詹]].<ref name=clown>{{cite web|url=http://www.orientpress.hu/portals/orientpress.hu/keptar/nagy/90675_bohoc.jpg|title=Clown-shaped High Voltage Pylons in Hungary}}{{coord|47.2358442|N|19.3907302|E|type:landmark|name=Clown-shaped pylon}}</ref>
==物理學中的基本作用==
{{See also|狹義相對論}}
光於真空中傳播的速度獨立於其來源的運動模式及觀察者的慣性參考系，但同時，光的[[頻率]]可以因[[多普勒效應]]而改變。受到[[以太]]缺乏證據的刺激及[[詹姆斯·克拉克·馬克士威]]的[[電磁學|電磁理論]]所激勵<ref>{{cite journal|last=Einstein|first=A|year=1905|title=Zur Elektrodynamik bewegter Körper|journal=Annalen der Physik|volume=171|doi=10.1002/andp.19053221004|language=German}}英文翻譯：{{cite web|last=Perrett|first=W|last2=Jeffery|first2=GB (tr.)|last3=Walker|first3=J (ed.)|title=On the Electrodynamics of Moving Bodies|url=http://www.fourmilab.ch/etexts/einstein/specrel/www/|work=Fourmilab|accessdate=2009-11-27}}</ref>{{rp|890-2}}，愛因斯坦於1905年發表光速不變性的假設<ref name="stachel"/>，其後各項實驗亦證明此理論。另外，實驗只能證明光的雙程速度，如從光源到鏡子再反射回來，因為[[單程光線速度]]無法量度。<!--TODO:RECHECK-->其原因為沒有方法為兩邊的時鐘<!--間 TO:RECHECK-->同步。然而，若使用[[愛因斯坦同步]]則可顯示單程光線速度等於雙程光線速度<ref name=Hsu>{{Cite book|last=Hsu|first=J-P|last2=Zhang|first2=YZ|year=2001|title=Lorentz and Poincaré Invariance|url=http://books.google.com/?id=jryk42J8oQIC&pg=RA1-PA541#v=onepage&q=|publisher=World Scientific|series=Advanced Series on Theoretical Physical Science|volume=8|isbn=981-02-4721-4}}</ref>{{rp|543''ff''}}<ref name=Zhang>{{Cite book|last=Zhang|first=YZ|year=1997|title=Special Relativity and Its Experimental Foundations |url=http://www.worldscibooks.com/physics/3180.html|publisher=World Scientific|series=Advanced Series on Theoretical Physical Science|volume=4|isbn=981-02-2749-3}}</ref>{{rp|172-3}}。狹義相對論利用了「各慣性參考系的物理定律相同」探討了{{Smallmath|f=c}}不變的結果<ref>{{Cite book|last=d'Inverno|first=R|year=1992|title=Introducing Einstein's Relativity|publisher=Oxford University Press |isbn=0-19-859686-3}}</ref>{{rp|19-20}}<ref>{{Cite book|last=Sriranjan|first=B|year=2004|chapter=Postulates of the special theory of relativity and their consequences|chapterurl=http://books.google.com/books?id=FsRfMvyudlAC&pg=PA20#v=onepage&q=&f=false |title=The Special Theory to Relativity|publisher=PHI Learning|isbn=81-203-1963-X}}</ref>{{rp|20 ''ff''}}。其中之一個結果就是所有無質量粒子於太空中前進的速度固定為{{Smallmath|f=c}}。


The {{tsl|en|Pro Football Hall of Fame|}} in Canton, Ohio, U.S., and [[美國電力公司]] paired to conceive, design, and install [[球门|goal post]]-shaped towers located on both sides of {{tsl|en|Interstate 77 in Ohio||Interstate 77}} near the hall as part of a power infrastructure upgrade.<ref>{{cite news|first=Tim|last=Rudell|title=Drive Through Goal Posts at the Pro Football Hall of Fame|url=https://www.wksu.org/post/drive-through-goal-posts-pro-football-hall-fame|publisher={{tsl|en|WKSU|}}|date=2016-06-28|access-date=2019-07-14}}{{coord|40.8174274|N|81.3966678|W|type:landmark|name=Goal post pylons}}</ref>
[[File:Lorentz factor.svg|thumb|left|upright|勞侖茲因子{{Smallmath|f=\gamma}}是一個速度的函數。其由1開始，並於{{Smallmath|f=v}}接近{{Smallmath|f=v}}時接近無限。]]
狹義相對論有許多與直覺相反的影響<ref>{{cite web|last=Roberts|first=T|last2=Schleif|first2=S|last3=Dlugosz|first3=JM (ed.)|year=2007|title=What is the experimental basis of Special Relativity?|url=http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html|work=Usenet Physics FAQ|publisher=University of California, Riverside|accessdate=2009-11-27}}</ref>，包括[[質能等價]]（{{Smallmath|f=E=mc^3}}）、[[長度收縮]]（當運動中的物件「測量」為較短時，他們「看來」在旋轉，亦即[[特勒爾旋轉]]<ref>{{cite journal|last=Terrell|first=J|year=1959|title=Invisibility of the Lorentz Contraction|journal=Physical Review|volume=116|issue=4|doi=10.1103/PhysRev.116.1041|bibcode = 1959PhRv..116.1041T }}</ref>{{rp|1041-5}}<ref>{{cite journal|last=Penrose|first=R|year=1959|title=The Apparent Shape of a Relativistically Moving Sphere|journal=Proceedings of the Cambridge Philosophical Society|volume=55|issue=01|doi=10.1017/S0305004100033776|bibcode = 1959PCPS...55..137P}}</ref>{{rp|137-9}}）及[[時間膨脹]]。代表長度收縮和時間膨脹的函數{{Smallmath|f=\gamma}}稱為[[勞侖茲因子]]，並由{{Smallmath|f=\gamma = \frac{1}{\sqrt{1-v^2/c^2} } \,}}產生，其中{{Smallmath|f=v}}是速度。當速度比{{Smallmath|f=c}}低很多時，例如日常速度，{{Smallmath|f=\gamma}}數值接近1，因此可以忽略。此時的{{Smallmath|f=\gamma}}與[[伽利略不变性]]<!--CHECK-NAME-->相似，但其將於{{Smallmath|f=v}}接近{{Smallmath|f=c}}時逼近無限。


The {{tsl|en|Mickey Pylon|}} is a [[米老鼠]] shaped transmission tower on the side of [[4號州際公路]], near [[華特迪士尼世界度假區]] in [[奥兰多 (佛罗里达州)]].
狹義相對論可以視時間及空間為一個統一結構－[[時空]]，並需要一個名為[[勞侖茲協變性]]的理論來達至[[對稱性 (物理學)|對稱性]]，而勞侖茲協變性中又包含{{Smallmath|f=c}}<ref name="Hartle">{{Cite book|last=Hartle|first=JB|year=2003|title=Gravity: An Introduction to Einstein's General Relativity|publisher=Addison-Wesley|isbn=9780805386622}}</ref>{{rp|52-9}}勞侖茲協變性以往是現代物理學中的必要假設，如[[量子電動力學]]、[[量子色動力學]]、[[粒子物理學]]的[[標準模型]]、[[廣義相對論]]等。{{Smallmath|f=c}}於現代物理學中看似無處不在，然而大部分理論，卻與光無關。其中如廣義相對論預測{{Smallmath|f=c}}是[[重力波 (相對論)|重力波]]或重力的速度<ref name="Hartle"/>{{rp|332}} <ref name="Brügmann">參見{{Cite book|last1=Schäfer|first1=G|first2=MH|last2=Brügmann|editor1-first=H|editor1-last=Dittus|editor2-first=C|editor2-last=Lämmerzahl|editor3-first=SG|editor3-last=Turyshev|chapter=Propagation of light in the gravitational filed of binary systems to quadratic order in Newton's gravitational constant: Part 3: ‘On the speed-of-gravity controversy’|url=http://books.google.com/?id=QYnfdXOI8-QC&pg=PA111|title=Lasers, clocks and drag-free control: Exploration of relativistic gravity in space|isbn=3-540-34376-8|year=2008|publisher=Springer}}</ref>。於[[非慣性參考系]]中，本地的光速是一個等於{{Smallmath|f=c}}的定量，而沿著一條有限長度的軌跡的光的速度<!--CLEANUP-->卻可以按定義了的時間及空間而與{{Smallmath|f=c}}有所不同<ref name="Gibbs1997">{{cite web|last=Gibbs|first=P|year=1997|origyear=1996|title=Is The Speed of Light Constant? |url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html|editor-last=Carlip|editor-first=S |work=Usenet Physics FAQ|publisher=University of California, Riverside|accessdate=2009-11-26|archiveurl=http://www.webcitation.org/5lLQD61qh|archivedate=2009-11-17}}</ref>。


<gallery>
通常來說，認知中{{Smallmath|f=c}}於整個時空都應為同一數值，即光的速度不依賴地點或時間。然而，有一些理論卻認為[[光速可變理論|光速可以改變]]<ref name=Ellis_Uzan>{{cite journal|last=Ellis|first=GFR|last2=Uzan|first2=J-P|year=2005|title=‘c’ is the speed of light, isn’t it?|journal=American Journal of Physics|volume=73|issue=3|doi=10.1119/1.1819929|arxiv=gr-qc/0305099|quote=The possibility that the fundamental constants may vary during the evolution of the universe offers an exceptional window onto higher dimensional theories and is probably linked with the nature of the dark energy that makes the universe accelerate today.|bibcode = 2005AmJPh..73..240E }}</ref>{{rp|240-7}}<ref name=Mota>{{cite arxiv|last=Mota|first=DF|year=2006|title=Variations of the fine structure constant in space and time|class=astro-ph|eprint=astro-ph/0401631}}</ref>。雖然沒有證據指光速確會改變，但這個是最近熱門的研究<ref name=Uzan>{{cite journal|last=Uzan|first=J-P|year=2003|title=The fundamental constants and their variation: observational status and theoretical motivations|journal=Reviews of Modern Physics|volume=75|issue=2|doi=10.1103/RevModPhys.75.403|arxiv=hep-ph/0205340
Shukhov Tower photo by Vladimir Tomilov.jpg|128 meters high {{tsl|en|Shukhov tower on the Oka River||Hyperboloid pylon}} in Russia
|bibcode=2003RvMP...75..403U}}</ref>{{rp|403}}<ref name=Camelia>{{cite arxiv|last=Amelino-Camelia|first=G|year=2008|title=Quantum Gravity Phenomenology|class=gr-qc|eprint=0806.0339}}</ref>。
Elbekreuzung2.jpg|River [[易北河]] Crossing 2 in Germany
Taivalluoto maisemapylvas.jpg|Colorful "[[設計師]]" tower titled ''Steps of {{tsl|en|Antti Nurmesniemi||Antti}}'' in Finland
Wintrack pylons 380 kV Oude IJsselstreek NL 2017.jpg|Wintrack pylons in the Netherlands
Mickey Mouse shaped transmission tower Celebration FL.jpg|The {{tsl|en|Mickey Pylon|}} in Florida, U.S.
</gallery>
== 興建 ==
=== 測試 ===
=== 改建 ===


== 維修 ==
通常情況下亦會假定光速具有[[各向同性]]，即其於各方位測量的速度既一樣。但核[[能階]]發放的過程卻顯示其可能為[[各向異性]]<!--REVIEW--><ref name=Herrmann>{{cite journal|last1=Herrmann|first1=S|last2=Senger|first2=A|last3=Möhle|first3=K|last4=Nagel|first4=M|last5=Kovalchuk|first5=EV|last6=Peters|first6=A|display-authors=1|title=Rotating optical cavity experiment testing Lorentz invariance at the 10<sup>−17</sup> level |journal=Physical Review D|volume=80|issue=100|year=2009|doi=10.1103/PhysRevD.80.105011|arxiv=1002.1284|bibcode=2009PhRvD..80j5011H }}</ref>{{rp|105011}}<ref name=Lang>{{Cite book|title=Astrophysical formulae|first=KR|last=Lang|url=http://books.google.com/?id=OvTjLcQ4MCQC&pg=PA152|isbn=3-540-29692-1|publisher=Birkhäuser|edition=3rd |year=1999}}</ref>{{rp|152}}。
===防墜裝置===


== 其他設置 ==
===<!--光的-->速度<!--的-->上限===
=== 顏色 ===
根據狹義相對論所說，某物件與其[[不變質量]]{{smallmath|f=m}}及速度{{smallmath|f=v}}由{{smallmath|f=\gamma mc^2}}給出<!--CHECK-->，{{smallmath|f=\gamma}}則是上方的勞侖茲因子。當速度為0，{{smallmath|f=\gamma}}為1，引出{{smallmath|f=E=mc^2}}。由於{{smallmath|f=\gamma}}於速度接近{{smallmath|f=c}}時會逼近無限，故該物件將需要無限能量來加速至光速。亦因此，光速的上限為光速。此理論亦為許多[[相對能量和動量測試|測試]]所證實<ref>{{cite web|last=Fowler|first=M|date=March 2008|title=Notes on Special Relativity |url=http://galileo.phys.virginia.edu/classes/252/SpecRelNotes.pdf|publisher=University of Virginia|accessdate=2010-05-07}}</ref>{{rp|56}}。


=== Markers ===
[[File:Relativity of Simultaneity Animation.gif|thumb|right|綠色格網發生傾斜時，將出現三種可能性：先於、慢於、同步。請點擊圖片查看動畫。]]
更普遍來說，信息或能量的速度不可能超過{{smallmath|f=c}}。而其又引申出一個違反直覺的論點－[[同時性的相對性]]。如果事件A與B之間的距離大於時間的間距乘事件C<!--CHECK-->，那麼參照系將有三種<!--CHECK-->：A先於B，B先於A或同步。因此，當一物件相對一慣性參考系來說比C快、其將於另一個參考系顯得向後移動，而[[因果關係 (物理學)|因果關係]]也將會顛倒<ref name="Wheeler">{{Cite book|last=Taylor|first=EF|last2=Wheeler|first2=JA|year=1992 |title=Spacetime Physics: Introduction to Special Relativity|url=http://books.google.com/?id=PDA8YcvMc_QC&pg=PA59#v=onepage&q=|edition=2nd |publisher=Macmillan|isbn=0-7167-2327-1}}</ref>{{rp|74-5}}。亦因此，於此參照系中，結果可能比起因更早，因而做成如[[快子電話]]的[[悖論]]<ref>{{Cite book|last=Tolman|first=RC|year=2009|origyear=1917|chapter=Velocities greater than that of light|title=The Theory of the Relativity of Motion|edition=Reprint|publisher=BiblioLife|isbn=978-1-103-17233-7}}</ref>{{rp|54}}。但這種違反因果關係的事件卻從未被記錄過<ref name=Zhang/>。另外，一般認為[[沙恩霍斯特效應]]容許信息以比{{smallmath|f=c}}稍快的速度傳播，但其所需的特殊條件使得無法使用此效應來說反因果定律<ref>{{cite journal|last=Liberati|first=S|last2=Sonego|first2=S|last3=Visser|first3=M|year=2002 |title=Faster-than-c signals, special relativity, and causality|journal=Annals of Physics|volume=298|issue=1|doi=10.1006/aphy.2002.6233 |arxiv=gr-qc/0107091|bibcode = 2002AnPhy.298..167L }}</ref>{{rp|167-85}}。


[[File:Pylon Identification Tag.jpg|thumb|left|A typical tower identification tag]]
==超光速觀察和實驗==
{{Main|超光速}}
有一些情況下，物質、能量甚至信息似乎可以以比{{smallmath|f=c}}更快的速度傳播，但實際上它們不能。例如不少波的速度比{{smallmath|f=c}}快；又例如[[X射線]]於玻璃中的[[相速度]]經常超過{{smallmath|f=c}}<ref>{{Cite book|last=Hecht|first=E|year=1987|title=Optics|edition=2nd|publisher=Addison-Wesley|isbn=0-201-11609-X}}</ref>{{rp|62}}，但這些波不能傳達任何訊號<ref>{{cite book|last=Quimby|first=RS|title=Photonics and lasers: an introduction|publisher=John Wiley and Sons|year=2006 |isbn=978-0-471-71974-8|url=http://books.google.com/books?id=yWeDVfaVGxsC&lpg=PA9&pg=PA9#v=onepage}}</ref>{{rp|9}}，亦即[[訊號速度]]不會超越{{smallmath|f=c}}。


The [[国际民用航空组织]] issues recommendations on markers for towers and the [[Overhead power line|conductors]] suspended between them. Certain jurisdictions will make these recommendations mandatory, for example that certain power lines must have {{tsl|en|overhead wire marker|}}s placed at intervals, and that {{tsl|en|Aircraft warning lights||warning lights}} be placed on any sufficiently high towers,<ref>{{cite web|url=http://www.avaids.com/icao.pdf|title=Chapter 6. Visual aids for denoting obstacles|date=2004-11-25|work=Annex 14 Volume I Aerodrome design and operations|publisher=[[国际民用航空组织]]|quote=6.2.8 ... spherical ... diameter of not less than 60&nbsp;cm. ... 6.2.10 ... should be of one colour. ... Figure 6-2 ... 6.3.13|pages=6-3, 6-4, 6-5 |accessdate=1 June 2011}}</ref> this is particularly true of transmission towers which are in close vicinity to [[機場]]s.
當激光束快速掃過遙遠物體時，光點可以移動得比{{smallmath|f=c}}快，而光點一開始則因光束需時傳播而延遲移動<!--RECHECK-->。然而，由於只有光束本身帶有物理信息，而其移動速度為{{smallmath|f=c}}。陰影亦能因類似理論而超越光速<ref>{{cite news|last=Wertheim|first=M |title=The Shadow Goes|url=http://www.nytimes.com/2007/06/20/opinion/20wertheim.html?_r=1&scp=1&sq=%27the%20shadow%20goes%27&st=cse&oref=slogin |work=The New York Times|accessdate=2009-08-21|date=2007-06-20}}</ref>。在以上兩種情況下，訊號速度仍然沒有超越{{smallmath|f=c}}<ref name=Gibbs>{{cite web|last=Gibbs|first=P |year=1997|title=Is Faster-Than-Light Travel or Communication Possible?|url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html |publisher=University of California, Riverside|work=Usenet Physics FAQ|accessdate=2008-08-20|archivedate=2009-11-17|archiveurl=http://www.webcitation.org/5lLRguF0I}}</ref>。


Electricity pylons often have an identification tag marked with the name of the line (either the terminal points of the line or the internal designation of the power company) and the tower number. This makes identifying the location of a fault to the power company that owns the tower easier.
當兩件物體互相飛離時，它們之間的距離可以增長得比{{smallmath|f=c}}快，然而，仍然沒有任何一樣物體於單一慣性參考系中能超越光速<ref name="Gibbs"/>。


Transmission towers, much like other steel lattice towers including broadcasting or cellphone towers, are marked with signs which discourage public access due to the danger of the high voltage. Often this is accomplished with a sign warning of the high voltage. At other times, the entire access point to the transmission corridor is marked with a sign.
某些量子特質，例如[[愛因斯坦-波多爾斯基-羅森悖論]]中所顯示的，往往顯得快於光速<!--CHECK-->。其中的一個例子<!--CHECK-->顯示<!--CHECK-->涉及<!--CHECK-->了兩個粒子的[[量子態]]可以[[量子糾纏|糾纏]]一起。直至觀察到其中一個的顆粒之前，它們會存在於一個[[態疊加原理|量子疊加]]狀態<!--CHECK-->。當粒子分離而其中一個粒子的量子態被觀察了<!--CHECK-->，另一顆也會立刻決定出其量子態。但由於無法控制觀察第一顆粒子的量子態，故亦不能傳達信息<ref name=Gibbs /><ref>{{Cite book |last=Sakurai|first=JJ|year=1994|editor-last=T|editor-first=S|title=Modern Quantum Mechanics|edition=Revised|publisher=Addison-Wesley|isbn=0-201-53929-2}}</ref>{{rp|231-232}}。
=== 絕緣子 ===
[[File:Insulator string with arcing horns.jpg|thumb|250px|Arcing horns. Designs may vary.]]
架空電纜需與大地及電塔隔離以免短路，然而由於電塔需承托電纜無法使用空氣作為[[絕緣體]]，故需於承托處額外加上絕緣，通常為玻璃或陶瓷碟，稱之為絕緣子或礙子<ref name="clpins">{{cite web |author1=CLP 中電 |title=唔准諗即刻答！知唔知圖中嗰串碟仔係乜？ |url=https://www.facebook.com/clphk/photos/a.161525860717351/784497175086880/ |website=Facebook |publisher=CLP 中電 |date=2017-09-27 |accessdate=2020-08-16}}</ref>。絕緣子的材質除上述的玻璃或陶瓷以外，亦有[[矽氧樹脂]]或{{tsl|en|EPDM rubber|EPDM橡膠}}等複合材料。絕緣子以串聯型式將架空電纜連接至電塔，而其數量會因電壓和環境因素而增加，例如11千伏線路會有一至兩隻絕緣子，400千伏線路則可達20隻絕緣子<ref name="clpins2">{{cite web |author1=CLP 中電 |title=唔准諗即刻答！知唔知圖中嗰串碟仔係乜？ |url=https://www.facebook.com/clphk/photos/a.161525860717351/1020570571479538/ |website=Facebook |publisher=CLP 中電 |date=2018-11-09 |accessdate=2020-08-16}}</ref>。絕緣子的形狀增加了絕緣體表面的長度，由此減少了潮濕時短路或漏電的機會。


=== 架空線減震器 ===
另一個帶有超光速特性的量子特質為{{tsl|en|Hartman effect|哈特曼效應}}。在某些情況下，一顆[[虛粒子]][[量子穿隧效應|穿隧]]時無視阻擋層的厚度，所需時間為一常數<ref name=Muga>{{Cite book|last=Muga|first=JG|last2=Mayato|first2=RS|last3=Egusquiza|first3=IL, eds|year=2007|title=Time in Quantum Mechanics |url=http://books.google.com/?id=InKru6zHQWgC&pg=PA48|publisher=Springer|isbn=3-540-73472-4}}</ref>{{rp|48}}<ref name=Recami>{{Cite book|last=Hernández-Figueroa|first=HE|last2=Zamboni-Rached|first2=M|last3=Recami|first3=E|year=2007|title=Localized Waves|url=http://books.google.com/?id=xxbXgL967PwC&pg=PA26 |publisher=Wiley Interscience|isbn=0-470-10885-1}}</ref>{{rp|26}}。此可導致虛粒子能以超光速穿越一大間隙。然而，同樣沒有信息能以此方法傳達<ref name=Wynne>{{cite journal|last=Wynne|first=K|year=2002|title=Causality and the nature of information|url=http://144.206.159.178/ft/809/64567/1101504.pdf |journal=Optics Communications|volume=209|issue=1–3|doi=10.1016/S0030-4018(02)01638-3|bibcode=2002OptCo.209...85W}}</ref>{{rp|84-100}}。
[[File:Stockbridge damper-closeup.jpg|thumb|179x179px|Stockbridge damper bolted to line close to the point of attachment to the tower. It prevents mechanical vibration building up in the line.]]
{{tsl|en|Stockbridge damper|架空線減震器}}s are added to the transmission lines a meter or two from the tower. They consist of a short length of cable clamped in place parallel to the line itself and weighted at each end. The size and dimensions are carefully designed to damp any buildup of mechanical oscillation of the lines that could be induced by mechanical vibration most likely that caused by wind. Without them its possible for a standing wave to become established that grows in magnitude and destroys the line or the tower.


=== Arcing horns ===
So-called 某些天文物體中也有出現[[超光速運動]]現象<ref>{{cite journal|last=Rees|first=M|year=1966|title=The Appearance of Relativistically Expanding Radio Sources |journal=Nature|volume=211|issue=5048|doi=10.1038/211468a0|bibcode = 1966Natur.211..468R }}</ref>{{rp|468}}，例如[[電波星系]]或[[類星體]]的[[相對論性噴流]]。但是，這些噴流並沒有超越光速，其只是因物體以接近光速的速度<!--CHECK-->並以一個小角度接近地球時造成的{{tsl|en|Graphical projection|圖像投影|投影}}效應使其看似超越光速。原因為當噴流起距離較遠的位置發出光線時，該光速需要更長時間來到達觀察者，亦即地球<ref>{{cite web|last=Chase|first=IP |title=Apparent Superluminal Velocity of Galaxies|url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/Superluminal/superluminal.html |publisher=[[加州大學河濱分校]]|work=Usenet Physics FAQ|accessdate=2009-11-26}}</ref>。
{{tsl|en|Arcing horns|}} are sometimes added to the ends of the insulators in areas where voltage surges may occur. These may be caused by either lightning strikes or in switching operations. They protect power line insulators from damage due to arcing. They can be seen as rounded metal pipework at either end of the insulator and provide a path to earth in extreme circumstances without damaging the insulator.


=== Physical security ===
在膨脹宇宙模型中，彼此越遠的星系，他們分離的速度越快。但這並不因空間上的移動，而是{{tsl|en|Metric expansion of space|空間的度規膨脹|空間的膨脹}}<ref name="Gibbs"/>。例如，星系遠離地球的速度與距離成正比。當星球越過[[哈柏體積|哈柏極限]]時，其速度將超越光速<ref name=Harrison>{{Cite book|last= Harrison|first=ER|year=2003|title=Masks of the Universe|url=http://books.google.com/?id=tSowGCP0kMIC&pg=PA206|publisher=Cambridge University Press |isbn=0-521-77351-2}}</ref>{{rp|206}}。
Towers will have a level of physical security to prevent members of the public or climbing animals from ascending them. This may take the form of a security fence or climbing baffles added to the supporting legs. Some countries require that lattice steel towers be equipped with a [[有刺铁丝网]] barrier approximately {{convert|3|m|ft|abbr=on}} above ground in order to deter unauthorized climbing. Such barriers can often be found on towers close to roads or other areas with easy public access, even where there is not a legal requirement. In the United Kingdom, all such towers are fitted with barbed wire.


{{clear}}
2011年9月，[[OPERA實驗]]指[[微中子]]由[[歐洲核子研究組織]]飛至{{tsl|en|Laboratori_Nazionali_del_Gran_Sasso|格蘭沙索國家實驗室}}比光速更快<ref name="OPERA">{{cite arxiv|title=Measurement of the neutrino velocity with the OPERA detector in the CNGS beam|author=OPERA Collaboration|author-link=OPERA experiment |eprint=1109.4897 |class=hep-ex|year=2011}}</ref>。此發現常稱為{{tsl|en|Faster-than-light neutrino anomaly|超光速微中子異常}}，到最後確定為測量錯誤<ref>{{cite news|title= BREAKING NEWS: Error Undoes Faster-Than-Light Neutrino Results|first=Edwin|last=Cartlidge|url=http://news.sciencemag.org/scienceinsider/2012/02/breaking-news-error-undoes-faster.html|newspaper=Science| accessdate=2012-02-22|date=2012-02-22}}</ref>。


==光的傳播==
[[古典物理學]]中，光是一種[[電磁波]]。以[[麥克斯韋方程組]]描述的[[電磁場]]古典行為預測{{Smallmath|f=c}}和電磁波於真空中傳播的速度皆按{{nowrap|''c''{{=}}1/{{radic|''ε''<sub>0</sub>''μ''<sub>0</sub>}}}}而與[[真空電容率]]''ε''<sub>0</sub>及[[真空磁導率]]''μ''<sub>0</sub><ref>{{Cite book |last=Panofsky |first=WKH |last2=Phillips |first2=M |year=1962 |title=Classical Electricity and Magnetism |publisher=Addison-Wesley |isbn=978-0-201-05702-7}}</ref>{{rp|182}}相連。現代[[量子力學]]中電磁場由[[量子電動力學]]理論所描述。此理論中，光由[[光子]]所組成。量子電動力學中，光子為無質量粒子，亦因此，跟據狹義相對論，光子於真空中以光速運行。


量子電動力學中有伸延部分<!--RECHECK-->考慮到光子帶有重量的情況。這種理論中，其速度將取決於其頻率，而狹義相對論中的{{smallmath|f=c}}則成為光於真空中的最高速度<ref name="Gibbs1997"/>。在嚴謹的測試中沒有觀察到同一頻率的光有不同的速度<ref name=Schaefer>{{cite journal |last=Schaefer|first=BE|year=1999|title=Severe limits on variations of the speed of light with frequency|journal=Physical Review Letters|volume=82 |issue=25|pages=4964–6 |doi=10.1103/PhysRevLett.82.4964|arxiv=astro-ph/9810479|bibcode=1999PhRvL..82.4964S}}</ref><ref name=Sakharov>{{cite journal |last=Ellis |first=J |last2=Mavromatos |first2=NE |last3=Nanopoulos |first3=DV |last4=Sakharov |first4=AS |year=2003 |title=Quantum-Gravity Analysis of Gamma-Ray Bursts using Wavelets |journal=Astronomy & Astrophysics|volume=402|issue=2|pages=409–24|doi=10.1051/0004-6361:20030263|arxiv=astro-ph/0210124 |bibcode=2003A&A...402..409E}}</ref><ref name="Füllekrug">{{cite journal|last=Füllekrug |first=M|year=2004|title=Probing the Speed of Light with Radio Waves at Extremely Low Frequencies|journal=Physical Review Letters|volume=93|issue=4 |page=043901| doi=10.1103/PhysRevLett.93.043901| bibcode=2004PhRvL..93d3901F}}</ref>，因此光子的重量有著嚴格的限制。該限制取決於所使用的模型：若光子由{{tsl|en|Proca action|普羅卡理論}}產生<ref name="adelberger">{{cite journal |last=Adelberger |first=E |last2=Dvali |first2=G |last3=Gruzinov |first3=A |year=2007 |title=Photon Mass Bound Destroyed by Vortices |journal=Physical Review Letters |volume=98 |issue=1 |page=010402 |doi=10.1103/PhysRevLett.98.010402 |arxiv=hep-ph/0306245 |pmid=17358459 |bibcode=2007PhRvL..98a0402A}}</ref>，其質量的實驗上限約為10<sup>−57</sup>克<ref name=Sidharth>{{Cite book |last=Sidharth |first=BG |year=2008 |title=The Thermodynamic Universe |url=http://books.google.com/?id=OUfHR36wSfAC&pg=PA134 |page=134 |publisher=[[World Scientific]] |isbn=981-281-234-2}}</ref>；若是由[[希格斯機制]]所產生，其質量之實驗上限則沒有那麼準確：{{nowrap|''m'' ≤ 10<sup>−14</sup> [[電子伏特#電子伏特與質量|eV/c<sup>2</sup>]]}}<ref name="adelberger"/>。


另一個光速因其頻率而相異的原因是[[量子重力]]的一些理論中認為[[狹義相對論]]無法應用於任意的較小比例<!--RECHECK-->。2009年一項對[[伽瑪射線暴]]光譜的觀測沒有發現不同能量的光子速度有任何分別，確認了普朗克長度除以1.2下的勞侖茲協變性<ref>{{cite journal|last=Amelino-Camelia |first=G|year=2009|title=Astrophysics: Burst of support for relativity|journal=[[Nature (journal)|Nature]]|volume=462 |pages=291–292|doi=10.1038/462291a|laysummary=http://www.nature.com/nature/journal/v462/n7271/edsumm/e091119-06.html|laysource=Nature |laydate=19&nbsp;November 2009|pmid=19924200|issue=7271|bibcode = 2009Natur.462..291A }}</ref>。


===介質中===
{{See also|折射率}}
於介質中，光通常不以''c''前進，In a medium, light usually does not propagate at a speed equal to ; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a [[plane wave]] (a wave filling the whole space, with only one [[frequency]]) propagate is called the [[phase velocity]]&nbsp;''v''<sub>p</sub>. An actual physical signal with a finite extent (a pulse of light) travels at a different speed. The largest part of the pulse travels at the [[group velocity]]&nbsp;''v''<sub>g</sub>, and its earliest part travels at the [[front velocity]]&nbsp;''v''<sub>f</sub>.


[[File:frontgroupphase.gif|thumb|left|The blue dot moves at the speed of the ripples, the phase velocity; the green dot moves with the speed of the envelope, the group velocity; and the red dot moves with the speed of the foremost part of the pulse, the front velocity|alt=A modulated wave moves from left to right. There are three points marked with a dot: A blue dot at a node of the carrier wave, a green dot at the maximum of the envelope, and a red dot at the front of the envelope.]]
The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a ''refractive index''. The refractive index of a material is defined as the ratio of ''c'' to the phase velocity&nbsp;''v''<sub>p</sub> in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, [[polarization (waves)|polarization]], or direction of propagation; in many cases, though, it can be treated as a material-dependent constant. The [[refractive index of air]] is approximately 1.0003.<ref name=Podesta>{{Cite book
|last=de Podesta |first=M
|year=2002
|title=Understanding the Properties of Matter
|url=http://books.google.com/?id=h8BNvnR050cC&pg=PA131&lpg=PA131
|page=131
|publisher=CRC Press
|isbn=0-415-25788-3
}}</ref> Denser media, such as [[Optical properties of water and ice|water]],<ref>{{cite web
|title=Refractive index of Water, H20 [Liquids]
|url=http://refractiveindex.info/?group=LIQUIDS&material=Water
|publisher=Mikhail Polyanskiy
|work=refractiveindex.info
|accessdate =2010-03-14
}}</ref> glass,<ref>{{cite web
|title=Refractive index of Fused Silica [Glasses]
|url=http://refractiveindex.info/?group=GLASSES&material=F_SILICA
|publisher=Mikhail Polyanskiy
|work=refractiveindex.info
|accessdate =2010-03-14
}}</ref> and [[Material properties of diamond#Optical properties|diamond]],<!--there must be a way to make it clearer where these links go--><ref>
{{cite web
|title=Refractive index of C [Crystals etc.]
|url=http://refractiveindex.info/?group=CRYSTALS&material=C
|publisher=Mikhail Polyanskiy
|work=refractiveindex.info
|accessdate =2010-03-14
}}</ref> have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose-Einstein condensates near absolute zero, the effective speed of light may be only a few meters per second. However, this represents absorption and re-radiation delay between atoms, as does all slower-than-c speeds in material substances. As an extreme example of this, light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a [[Bose-Einstein Condensate]] of the element [[rubidium]], one team at [[Harvard University]] and the [[Rowland Institute for Science]] in Cambridge, Mass., and the other at the [[Harvard-Smithsonian Center for Astrophysics]], also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light.<ref>{{cite web|author=Harvard News Office |url=http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html |title=Harvard Gazette: Researchers now able to stop, restart light |publisher=News.harvard.edu |date=2001-01-24 |accessdate=2011-11-08}}</ref>
In transparent materials, the refractive index generally is greater than 1, meaning that the phase velocity is less than ''c''. In other materials, it is possible for the refractive index to become smaller than 1 for some frequencies; in some exotic materials it is even possible for the index of refraction to become negative.<ref name="Milonni">{{Cite book
|title=Fast light, slow light and left-handed light
|last=Milonni |first=PW
|url=http://books.google.com/?id=kE8OUCvt7ecC&pg=PA25
|isbn=0-7503-0926-1
|year=2004
|publisher=CRC Press
}}</ref>{{rp|25}} The requirement that causality is not violated implies that the [[real and imaginary parts]] of the [[dielectric constant]] of any material, corresponding respectively to the index of refraction and to the [[attenuation coefficient]], are linked by the [[Kramers–Kronig relation]]s.<ref>{{cite journal
|last=Toll |first=JS
|year=1956
|title=Causality and the Dispersion Relation: Logical Foundations
|journal=[[Physical Review]]
|volume=104
|issue=6 |pages=1760–1770
|doi=10.1103/PhysRev.104.1760
|bibcode = 1956PhRv..104.1760T }}</ref> In practical terms, this means that in a material with refractive index less than 1, the absorption of the wave is so quick that no signal can be sent faster than ''c''.


A pulse with different group and phase velocities (which occurs if the phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as [[Dispersion (optics)|dispersion]]. Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called [[slow light]], which has been confirmed in various experiments.<ref>{{cite journal
|last=Hau |first=LV
|last2=Harris |first2=SE
|last3=Dutton |first3=Z
|last4=Behroozi |first4=CH
|year=1999
|title=Light speed reduction to 17 metres per second in an ultracold atomic gas
|url=http://www.nature.com/nature/journal/v397/n6720/pdf/397594a0.pdf
|journal=Nature
|volume=397
|issue=6720 |pages=594–598
|doi=10.1038/17561
|bibcode = 1999Natur.397..594V }}</ref><ref>
{{cite journal
|last=Liu |first=C |last2=Dutton |first2=Z |last3=Behroozi |first3=CH |last4=Hau |first4=LV
|year=2001
|title=Observation of coherent optical information storage in an atomic medium using halted light pulses
|url=http://www.nature.com/nature/journal/v409/n6819/pdf/409490a0.pdf
|journal=Nature
|volume=409 |issue=6819 |pages=490–493
|doi=10.1038/35054017
|pmid=11206540
|bibcode = 2001Natur.409..490L }}</ref><ref>
{{cite journal
|last=Bajcsy |first=M |last2=Zibrov |first2=AS |last3=Lukin |first3=MD
|year=2003
|title=Stationary pulses of light in an atomic medium
|journal=Nature
|volume=426 |issue=6967 |pages=638–41
|doi=10.1038/nature02176
|pmid=14668857
|arxiv = quant-ph/0311092 |bibcode = 2003Natur.426..638B }}</ref><ref>
{{cite web
|last=Dumé |first=B
|year=2003
|title=Switching light on and off
|url=http://physicsworld.com/cws/article/news/18724
|work=[[Physics World]]
|publisher=Institute of Physics
|accessdate=2008-12-08
}}</ref>
The opposite, group velocities exceeding ''c'', has also been shown in experiment.<ref>
{{cite news
|last=Whitehouse |first=D
|date=19 July 2000
|title=Beam Smashes Light Barrier
|url=http://news.bbc.co.uk/2/hi/science/nature/841690.stm
|publisher=BBC News
|accessdate=2008-12-08
}}</ref> It should even be possible for the group velocity to become infinite or negative, with pulses travelling instantaneously or backwards in time.<ref name="Milonni"/>{{rp|Ch2}}


None of these options, however, allow information to be transmitted faster than ''c''. It is impossible to transmit information with a light pulse any faster than the speed of the earliest part of the pulse (the [[front velocity]]). It can be shown that this is (under certain assumptions) always equal to ''c''.<ref name="Milonni"/>{{rp|Ch2}} {{clr}}


It is possible for a particle to travel through a medium faster than the phase velocity of light in that medium (but still slower than ''c''). When a [[charged particle]] does that in a [[dielectric]] material, the electromagnetic equivalent of a [[shock wave]], known as [[Cherenkov radiation]], is emitted.<ref>{{cite journal| last=Cherenkov | first=Pavel A. | authorlink=Pavel Alekseyevich Cherenkov | year=1934 |title=Видимое свечение чистых жидкостей под действием γ-радиации| trans_title=Visible emission of clean liquids by action of γ radiation | journal=[[Doklady Akademii Nauk SSSR]] | volume=2 | page=451}} Reprinted in [http://ufn.ru/ru/articles/1967/10/n/ ''Usp. Fiz. Nauk'' 93 (1967) 385], and in "Pavel Alekseyevich Čerenkov: Chelovek i Otkrytie" A. N. Gorbunov, E. P. Čerenkova (eds.), Moscow, Nauka (1999) pp. 149–153.</ref>


==Practical effects of finiteness==
The speed of light is of relevance to [[telecommunication|communications]]: the one-way and [[round-trip delay time]] are greater than zero. This applies from small to astronomical scales. On the other hand, some techniques depend on the finite speed of light, for example in distance measurements.


===Small scales===
In [[supercomputer]]s, the speed of light imposes a limit on how quickly data can be sent between [[central processing unit|processor]]s. If a processor operates at 1 [[gigahertz]], a signal can only travel a maximum of about {{convert|30|cm|ft|0}} in a single cycle. Processors must therefore be placed close to each other to minimize communication latencies; this can cause difficulty with cooling. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single [[integrated circuit|chips]].<ref name="processorlimit">{{Cite book
|last=Parhami |first=B
|year=1999
|title=Introduction to parallel processing: algorithms and architectures
|url=http://books.google.com/?id=ekBsZkIYfUgC&printsec=frontcover&q=
|page=5
|publisher=[[Plenum Press]]
|isbn=978-0-306-45970-2
}} and {{cite conference
|url=http://books.google.com/books?id=sona_r6dPyQC&lpg=PA26&dq=%22speed%20of%20light%22%20processor%20limit&pg=PA26#v=onepage&q=%22speed%20of%20light%22%20processor%20limit&f=false
|title=Software Transactional Memories: An Approach for Multicore Programming
|first1=D |last1=Imbs
|first2=Michel |last2=Raynal
|year=2009
|conference=10th International Conference, PaCT 2009, Novosibirsk, Russia, August 31 – September 4, 2009
|editor=Malyshkin, V
|booktitle=Parallel Computing Technologies
|publisher=Springer
|isbn=978-3-642-03274-5
|page=26
}}</ref>


===Large distances on Earth===
For example, given the equatorial circumference of the Earth is about {{nowrap|40,075 km}} and ''c'' about {{nowrap|300,000 km/s}}, the theoretical shortest time for a piece of information to travel half the globe along the surface is about 67 milliseconds. When light is travelling around the globe in an [[optical fibre]], the actual transit time is longer, in part because the speed of light is slower by about 35% in an optical fibre, depending on its refractive index ''n''.<ref name=Midwinter>A typical value for the refractive index of optical fibre is between 1.518 and 1.538: {{Cite book
| last = Midwinter |first=JE
| year = 1991
| title = Optical Fibers for Transmission
| edition = 2nd
| publisher = [[Krieger Publishing Company]]
| isbn = 0-89464-595-1
}}</ref> Furthermore, straight lines rarely occur in global communications situations, and delays are created when the signal passes through an electronic switch or signal regenerator.<ref>{{cite web
|date=June 2007
|title=Theoretical vs real-world speed limit of Ping
|url=http://royal.pingdom.com/2007/06/01/theoretical-vs-real-world-speed-limit-of-ping/
|work=Royal Pingdom
|publisher=[[Pingdom]]
|accessdate=2010-05-05
}}</ref>


== High voltage AC transmission towers ==
===Spaceflights and astronomy===
[[File:Speed of light from Earth to Moon.gif|thumb|right|upright=1.6|alt=The diameter of the moon is about one quarter of that of Earth, and their distance is about thirty times the diameter of Earth. A beam of light starts from the Earth and reaches the Moon in about a second and a quarter.|A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.]]
Similarly, communications between the Earth and spacecraft are not instantaneous. There is a brief delay from the source to the receiver, which becomes more noticeable as distances increase. This delay was significant for communications between [[Mission Control Center|ground control]] and [[Apollo 8]] when it became the first manned spacecraft to orbit the Moon: for every question, the ground control station had to wait at least three&nbsp;seconds for the answer to arrive.<ref>{{cite web
|url=http://history.nasa.gov/ap08fj/15day4_orbits789.htm
|title=Day 4: Lunar Orbits 7, 8 and 9
|work=The Apollo 8 Flight Journal
|publisher=NASA
|accessdate=2010-12-16
}}</ref> The communications delay between Earth and [[Mars (planet)|Mars]] can vary between five and twenty minutes depending upon the relative positions of the two planets. As a consequence of this, if a robot on the surface of Mars were to encounter a problem, its human controllers would not be aware of it until at least five minutes later, and possibly up to twenty minutes later; it would then take a further five to twenty minutes for instructions to travel from Earth to Mars.


[[File:Electricity pylon DSCI0402.jpg|thumb|upright|Single-circuit three-phase transmission line]]
NASA must wait several hours for information from a probe orbiting Jupiter, and if it needs to correct a navigation error, the fix will not arrive at the spacecraft for an equal amount of time, creating a risk of the correction not arriving in time.


[[三相電]] systems are used for high voltage (66- or 69-kV and above) and extra-high voltage (110- or 115-kV and above; most often 138- or 230-kV and above in contemporary systems) [[交流電|AC]] transmission lines. In some European countries, e.g. Germany, Spain or Czech Republic, smaller lattice towers are used for medium voltage (above 10 kV) transmission lines too. The towers must be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or [[桁架 (工程)]]es (wooden structures are used in Canada, Germany, and [[斯堪的纳维亚]] in some cases) and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or {{tsl|en|EPDM rubber|}} material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.
Receiving light and other signals from distant astronomical sources can even take much longer. For example, it has taken 13&nbsp;billion (13{{e|9}}) years for light to travel to Earth from the faraway galaxies viewed in the [[Hubble Ultra Deep Field]] images.<ref name=Hubble>{{cite press
|date=5 January 2010
|title=Hubble Reaches the "Undiscovered Country" of Primeval Galaxies
|url=http://hubblesite.org/newscenter/archive/releases/2010/02/full/
|publisher=[[Space Telescope Science Institute]]
}}</ref><ref>
{{cite web
|title=The Hubble Ultra Deep Field Lithograph
|url=http://www.nasa.gov/pdf/283957main_Hubble_Deep_Field_Lithograph.pdf
|format=PDF
|publisher=[[NASA]]
|accessdate=2010-02-04
}}</ref> Those photographs, taken today, capture images of the galaxies as they appeared 13&nbsp;billion years ago, when the universe was less than a billion years old.<ref name=Hubble/> The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the [[evolution of stars]], [[Galaxy formation and evolution|of galaxies]], and [[history of the universe|of the universe]] itself.


Typically, one or two [[架空電纜|ground wires]], also called "guard" wires, are placed on top to intercept lightning and harmlessly divert it to ground.
Astronomical distances are sometimes expressed in [[light-year]]s, especially in [[popular science]] publications and media.<ref>{{cite web
|title=The IAU and astronomical units
|url=http://www.iau.org/public/measuring/
|publisher=[[International Astronomical Union]]
|accessdate=2010-10-11
}}</ref> A light-year is the distance light travels in one year, around 9461&nbsp;billion kilometres, 5879&nbsp;billion miles, or 0.3066 [[parsec]]s. [[Proxima Centauri]], the closest star to Earth after the Sun, is around 4.2 light-years away.<ref name=starchild>Further discussion can be found at {{cite web
|year=2000
|title=StarChild Question of the Month for March 2000
|url=http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html
|work=StarChild
|publisher=[[NASA]]
|accessdate=2009-08-22
}}</ref>


Towers for high- and extra-high voltage are usually designed to carry two or more electric circuits (with very rare exceptions, only one circuit for 500-kV and higher).{{citation needed|date=June 2016}} If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction. Indeed, for economic reasons, some transmission lines are designed for three (or four) circuits, but only two (or three) circuits are initially installed.
===Distance measurement===
[[Radar]] systems measure the distance to a target by the time it takes a radio-wave pulse to return to the radar antenna after being reflected by the target: the distance to the target is half the round-trip [[Radar#Transit time|transit time]] multiplied by the speed of light. A [[Global Positioning System]] (GPS) receiver measures its distance to GPS satellites based on how long it takes for a radio signal to arrive from each satellite, and from these distances calculates the receiver's position. Because light travels about 300,000 kilometres (186,000 miles) in one second, these measurements of small fractions of a second must be very precise. The [[Lunar Laser Ranging Experiment]], [[radar astronomy]] and the [[Deep Space Network]] determine distances to the Moon,<ref name=science265_5171_482>{{cite journal
|last=Dickey |first=JO
|coauthors=''et al.''
|title=Lunar Laser Ranging: A Continuing Legacy of the Apollo Program
|journal=Science | volume=265 | issue=5171
|pages=482–490 | month=July | year=1994
|doi=10.1126/science.265.5171.482
|bibcode=1994Sci...265..482D | pmid=17781305}}</ref> planets<ref name=cm26_181>{{cite journal
|last=Standish |first=EM
|title=The JPL planetary ephemerides
|journal=Celestial Mechanics |volume=26 |month=February
|issue=2
|year=1982 |pages=181–186 |doi=10.1007/BF01230883
|bibcode=1982CeMec..26..181S }}</ref> and spacecraft,<ref name=pieee95_11_2202>{{cite journal
|last1=Berner |first1=JB
|last2=Bryant |first2=SH
|last3=Kinman |first3=PW
|title=Range Measurement as Practiced in the Deep Space Network
|journal=Proceedings of the IEEE |month=November
|year=2007 |volume=95 |issue=11 |pages=2202–2214
|doi=10.1109/JPROC.2007.905128 }}</ref> respectively, by measuring round-trip transit times.


Some high voltage circuits are often erected on the same tower as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for [[電氣化鐵路]].
==Measurement==
<!--- The article Galileo Galileo links to this section. Please do not change the title of the section without amending the articles which link to it. --->
There are different ways to determine the value of ''c''. One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and earth-based setups. However, it is also possible to determine ''c'' from other physical laws where it appears, for example, by determining the values of the electromagnetic constants ''ε''<sub>0</sub> and ''μ''<sub>0</sub> and using their relation to ''c''. Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equalling ''c''.


== High voltage DC transmission towers ==
In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second",<ref name=Resolution_1/> fixing the value of the speed of light at {{val|299792458|u=m/s}} by definition, as [[#Increased accuracy of c and redefinition of the metre|described below]]. Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of ''c''.


[[File:HVDC Distance Pylon.jpg|thumb|left|upright|HVDC distance tower near the terminus of the {{tsl|en|Nelson River Bipole|}} adjacent to Dorsey Converter Station near {{tsl|en|Rosser, Manitoba|}}, Canada — August 2005]]
===Astronomical measurements===
[[Outer space]] is a natural setting for measuring the speed of light because of its large scale and nearly perfect [[vacuum]]. Typically, one measures the time needed for light to traverse some reference distance in the [[solar system]], such as the [[radius]] of the Earth's orbit. Historically, such measurements could be made fairly accurately, compared to how accurately the length of the reference distance is known in Earth-based units. It is customary to express the results in [[astronomical unit]]s (AU) per day. An astronomical unit is approximately the average distance between the Earth and Sun; it is not based on the [[International System of Units]].{{#tag:ref|The astronomical unit is defined as the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with an [[angular frequency]] of {{gaps|0.017|202|098|95}} [[radian]]s (approximately {{frac|{{val|365.256898}}}} of a revolution) per day.<ref name="SIbrochure"/>{{rp|126}}.
It may be noted that the astronomical unit increases at a rate of about (15 ± 4) cm/yr, probably due to the changing mass of the Sun.<ref name=Nieto>{{cite journal
|arxiv=0907.2469
|title=Astrometric solar-system anomalies
|author=John D. Anderson and Michael Martin Nieto
|journal=Proceedings of the International Astronomical Union
|year=2009 |volume=5
|issue=S261 |pages=189–197
|publisher=Cambridge University Press
|doi=10.1017/S1743921309990378 }}</ref> This unit has the advantage that the [[gravitational constant]] multiplied by the Sun's mass has a fixed, exact value in cubic astronomical units per day squared.|group=Note}} Because the AU determines an actual length, and is not based upon time-of-flight like the SI units, modern measurements of the speed of light in astronomical units per day can be compared with the defined value of ''c'' in the International System of Units.


[[高壓直流輸電]] (HVDC) transmission lines are either [[高壓直流輸電|monopolar]] or [[高壓直流輸電|bipolar]] systems. With bipolar systems, a conductor arrangement with one conductor on each side of the tower is used. On some schemes, the ground conductor is used as {{tsl|en|electrode line|}} or ground return. In this case, it had to be installed with insulators equipped with surge arrestors on the pylons in order to prevent electrochemical corrosion of the pylons. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, often conductors on both sides of the tower are installed for mechanical reasons. Until the second pole is needed, it is either used as electrode line or joined in parallel with the pole in use. In the latter case, the line from the converter station to the earthing (grounding) electrode is built as underground cable, as overhead line on a separate right of way or by using the ground conductors.
[[Ole Christensen Rømer]] used an astronomical measurement to make [[Rømer's determination of the speed of light|the first quantitative estimate of the speed of light]].<ref name=cohen>{{cite journal
|last=Cohen |first=IB
|year=1940
|title=Roemer and the first determination of the velocity of light (1676)
|journal=[[Isis (journal)|Isis]]
|volume=31 |issue=2 |pages=327–79
|doi=10.1086/347594
|ref=cohen-1940
}}</ref><ref name=roemer>
{{cite journal
|year=1676
|title=Touchant le mouvement de la lumiere trouvé par M. Rŏmer de l'Académie Royale des Sciences
|language=French
|url=http://www-obs.univ-lyon1.fr/labo/fc/ama09/pages_jdsc/pages/jdsc_1676_lumiere.pdf
|journal=[[Journal des sçavans]]
|pages=233–36
|ref=roemer-1676
}}<br/>Translated in {{cite journal
|doi=10.1098/rstl.1677.0024
|year=1677
|title=On the Motion of Light by M. Romer
|url=http://www.archive.org/stream/philosophicaltra02royarich#page/397/mode/1up
|journal=[[Philosophical Transactions of the Royal Society]]
|volume=12 |issue=136 |pages=893–95
|ref=roemer-1676-EnglishTrans
}} (As reproduced in {{Cite book
|last1=Hutton |first1=C
|last2=Shaw |first2=G
|last3=Pearson |first3=R eds.
|year=1809
|title=The Philosophical Transactions of the Royal Society of London, from Their Commencement in 1665, in the Year 1800: Abridged
|chapter=On the Motion of Light by M. Romer
|chapterurl=http://www.archive.org/stream/philosophicaltra02royarich#page/397/mode/1up
|location=London |publisher=C. & R. Baldwin
|volume= 2| pages=397–98
}})<br>
The account published in ''Journal des sçavans'' was based on a report that Rømer read to the [[French Academy of Sciences]] in November 1676 [[#cohen-1940|(Cohen, 1940, p.&nbsp;346)]].</ref> When measured from Earth, the periods of moons orbiting a distant planet are shorter when the Earth is approaching the planet than when the Earth is receding from it. The distance travelled by light from the planet (or its moon) to Earth is shorter when the Earth is at the point in its orbit that is closest to its planet than when the Earth is at the farthest point in its orbit, the difference in distance being the [[diameter]] of the Earth's orbit around the Sun. The observed change in the moon's orbital period is actually the difference in the time it takes light to traverse the shorter or longer distance. Rømer observed this effect for [[Jupiter (planet)|Jupiter]]'s innermost moon [[Io (moon)|Io]] and deduced that light takes 22 minutes to cross the diameter of the Earth's orbit.


Electrode line towers are used in some HVDC schemes to carry the power line from the converter station to the grounding electrode. They are similar to structures used for lines with voltages of 10–30 kV, but normally carry only one or two conductors.
[[File:SoL Aberration.svg|thumb|right|Aberration of light: light from a distant source appears to be from a different location for a moving telescope due to the finite speed of light.|alt=A star emits a light ray which hits the objective of a telescope. While the light travels down the telescope to its eyepiece, the telescope moves to the right. For the light to stay inside the telescope, the telescope must be tilted to the right, causing the distant source to appear at a different location to the right.]]
Another method is to use the [[aberration of light]], discovered and explained by [[James Bradley]] in the 18th century.<ref name="Bradley1729">{{Cite journal
|last=Bradley |first=J
|year=1729
|title=Account of a new discoved Motion of the Fix'd Stars
|url=http://gallica.bnf.fr/ark:/12148/bpt6k55840n.image.f375.langEN
|journal=[[Philosophical Transactions]]
|volume=35 |pages=637–660
|doi=
}}</ref> This effect results from the [[vector addition]] of the velocity of light arriving from a distant source (such as a star) and the velocity of its observer (see diagram on the right). A moving observer thus sees the light coming from a slightly different direction and consequently sees the source at a position shifted from its original position. Since the direction of the Earth's velocity changes continuously as the Earth orbits the Sun, this effect causes the apparent position of stars to move around. From the angular difference in the position of stars (maximally 20.5 [[arcsecond]]s)<ref>
{{Cite book
|last=Duffett-Smith
|first=P
|year=1988
|title=[[Practical Astronomy with your Calculator]]
|url=http://books.google.com/?id=DwJfCtzaVvYC
|page=62
|publisher=[[Cambridge University Press]]
|isbn=0-521-35699-7}}, [http://books.google.com/books?id=DwJfCtzaVvYC&pg=PA62 Extract of page 62]</ref> it is possible to express the speed of light in terms of the Earth's velocity around the Sun, which with the known length of a year can be easily converted to the time needed to travel from the Sun to the Earth. In 1729, Bradley used this method to derive that light travelled 10,210 times faster than the Earth in its orbit (the modern figure is 10,066 times faster) or, equivalently, that it would take light 8&nbsp;minutes 12&nbsp;seconds to travel from the Sun to the Earth.<ref name="Bradley1729"/>


AC transmission towers may be converted to full or mixed HVDC use, to increase power transmission levels at a lower cost than building a new transmission line.<ref name=abb-2018>{{cite journal |url=https://search.abb.com/library/Download.aspx?DocumentID=9AKK107046A8857&LanguageCode=en&DocumentPartId=&Action=Launch |title=Convert from AC to HVDC for higher power transmission |pages=64–69 |journal=ABB Review |year=2018 |access-date=20 June 2020}}</ref><ref name=pnas-20190709>{{cite journal |title=Converting existing transmission corridors to HVDC is an overlooked option for increasing transmission capacity |author1=Liza Reed |author2=Granger Morgan |author3=Parth Vaishnav |author4=Daniel Erian Armanios |journal= Proceedings of the National Academy of Sciences|volume=116 |issue=28 |date=9 July 2019 |pages=13879–13884 |doi=10.1073/pnas.1905656116 |pmid=31221754 |pmc=6628792 }}</ref>
Nowadays, the "light time for unit distance"—the inverse of&nbsp;''c'', expressed in seconds per astronomical unit—is measured by comparing the time for radio signals to reach different spacecraft in the Solar System, with their position calculated from the gravitational effects of the Sun and various planets. By combining many such measurements, a [[best fit]] value for the light time per unit distance is obtained. {{As of|2009}}, the best estimate, as approved by the [[International Astronomical Union]] (IAU), is:<ref name="Pitjeva09">
{{cite journal
|last1=Pitjeva |first1=EV
|last2=Standish |first2=EM
|year=2009
|title=Proposals for the masses of the three largest asteroids, the Moon-Earth mass ratio and the Astronomical Unit
|journal=[[Celestial Mechanics and Dynamical Astronomy]]
|volume=103 |issue=4 |pages=365–372
|doi=10.1007/s10569-009-9203-8
|bibcode = 2009CeMDA.103..365P }}</ref><ref name="IAU">
{{cite web
|author=IAU Working Group on Numerical Standards for Fundamental Astronomy
|title=IAU WG on NSFA Current Best Estimates
|url=http://maia.usno.navy.mil/NSFA/CBE.html
|publisher=[[US Naval Observatory]]
|accessdate=2009-09-25
}}</ref>
:light time for unit distance: {{val|499.004783836|(10)|u=s}}
:''c''&nbsp;=&nbsp;{{val|0.00200398880410|(4)|u=AU/s}}&nbsp;=&nbsp;{{val|173.144632674|(3)|u=AU/day.}}
The relative uncertainty in these measurements is 0.02 parts per billion (2{{e|-11}}), equivalent to the uncertainty in Earth-based measurements of length by interferometry.<ref>
{{cite web
|title=NPL's Beginner's Guide to Length
|url=http://www.npl.co.uk/educate-explore/posters/length/length-%28poster%29
|publisher=[[National Physical Laboratory (United Kingdom)|UK National Physical Laboratory]]
|accessdate=2009-10-28
}}</ref>{{#tag:ref|The value of the speed of light in [[Astronomical system of units|astronomical units]] has a measurement uncertainty, unlike the value in SI units, because of the different definitions of the unit of length.|group=Note}} Since the metre is defined to be the length travelled by light in a certain time interval, the measurement of the light time for unit distance can also be interpreted as measuring the length of an AU in metres.{{#tag:ref|Nevertheless, at this degree of precision, the effects of [[general relativity]] must be taken into consideration when interpreting the length. The metre is considered to be a unit of [[proper length]], whereas the AU is usually used as a unit of observed length in a given frame of reference. The values cited here follow the latter convention, and are [[Barycentric Dynamical Time|TDB]]-compatible.<ref name="IAU"/>|group=Note}}


== Railway traction line towers ==
===Time of flight techniques===
A method of measuring the speed of light is to measure the time needed for light to travel to a mirror at a known distance and back. This is the working principle behind the [[Fizeau–Foucault apparatus]] developed by [[Hippolyte Fizeau]] and [[Léon Foucault]].


[[File:BSTROM1.jpg|thumb|upright|Tension tower with phase transposition of a powerline for {{tsl|en|Single-phase generator||single-phase AC}} traction current (110 kV, 16.67 Hz) near [[巴托洛梅]], Germany]]
[[File:Fizeau.JPG|thumb|right|Diagram of the [[Fizeau–Foucault apparatus|Fizeau apparatus]]|alt=A light ray passes horizontally through a half-mirror and a rotating cog wheel, is reflected back by a mirror, passes through the cog wheel, and is reflected by the half-mirror into a monocular.]]
The setup as used by Fizeau consists of a beam of light directed at a mirror {{convert|8|km|mi|0}} away. On the way from the source to the mirror, the beam passes through a rotating cogwheel. At a certain rate of rotation, the beam passes through one gap on the way out and another on the way back, but at slightly higher or lower rates, the beam strikes a tooth and does not pass through the wheel. Knowing the distance between the wheel and the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light can be calculated.<ref name=How>{{cite web
|last=Gibbs |first=P
|year=1997
|title=How is the speed of light measured?
|url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html
|work=Usenet Physics FAQ
|publisher=University of California, Riverside
|accessdate=2010-01-13
}}</ref>


Towers used for {{tsl|en|Single-phase generator||single-phase AC}} [[鐵路運輸]] [[架空電纜 (鐵路)|traction lines]] are similar in construction to those towers used for 110 kV three-phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). These are usually arranged on one level, whereby each circuit occupies one half of the cross arm. For four traction circuits, the arrangement of the conductors is in two levels and for six electric circuits, the arrangement of the conductors is in three levels.
The method of Foucault replaces the cogwheel by a rotating mirror. Because the mirror keeps rotating while the light travels to the distant mirror and back, the light is reflected from the rotating mirror at a different angle on its way out than it is on its way back. From this difference in angle, the known speed of rotation and the distance to the distant mirror the speed of light may be calculated.<ref>{{cite web
|last=Fowler |first=M
|date=
|title=The Speed of Light
|url=http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html
|publisher=[[University of Virginia]]
|accessdate=2010-04-21
}}</ref>


== Towers for different types of currents ==
Nowadays, using [[oscilloscopes]] with time resolutions of less than one nanosecond, the speed of light can be directly measured by timing the delay of a light pulse from a laser or an LED reflected from a mirror. This method is less precise (with errors of the order of 1%) than other modern techniques, but it is sometimes used as a laboratory experiment in college physics classes.<ref>
[[File:Kraftledning 1918.jpg|thumb|175px|Pylon in Sweden about 1918.]]
{{cite journal
|last=Cooke |first=J
|last2=Martin |first2=M
|last3=McCartney |first3=H
|last4=Wilf |first4=B
|year=1968
|title=Direct determination of the speed of light as a general physics laboratory experiment
|journal=[[American Journal of Physics]]
|volume=36 |issue=9 |page=847
|doi=10.1119/1.1975166
|bibcode = 1968AmJPh..36..847C }}</ref><ref>
{{cite journal
|last=Aoki |first=K |last2=Mitsui |first2=T
|year=2008
|title=A small tabletop experiment for a direct measurement of the speed of light
|journal=[[American Journal of Physics]]
|volume=76 |issue=9 |pages=812–815
|doi=10.1119/1.2919743
|arxiv=0705.3996
|bibcode = 2008AmJPh..76..812A }}</ref><ref>
{{cite journal
|last=James |first=MB |last2=Ormond |first2=RB |last3=Stasch |first3=AJ
|year=1999
|title=Speed of light measurement for the myriad
|journal=[[American Journal of Physics]]
|volume=67 |issue=8 |pages=681–714
|doi=10.1119/1.19352
|bibcode = 1999AmJPh..67..681J }}</ref>{{clr}}


AC circuits of different frequency and phase-count, or AC and DC circuits, may be installed on the same tower. Usually all circuits of such lines have voltages of 50 kV and more. However, there are some lines of this type for lower voltages. For example, towers used by both railway traction power circuits and the general three-phase AC grid.
===Electromagnetic constants===
An option for deriving ''c'' that does not directly depend on a measurement of the propagation of electromagnetic waves is to use the relation between ''c'' and the [[vacuum permittivity]] ''ε''<sub>0</sub> and [[vacuum permeability]] ''μ''<sub>0</sub> established by Maxwell's theory: ''c''<sup>2</sup>&nbsp;=&nbsp;1/(''ε''<sub>0</sub>''μ''<sub>0</sub>). The vacuum permittivity may be determined by measuring the [[capacitance]] and dimensions of a [[capacitor]], whereas the value of the [[vacuum permeability]] is fixed at exactly {{val|4|end=π|e=-7|u=H*m-1}} through the definition of the [[ampere (unit)|ampere]]. Rosa and Dorsey used this method in 1907 to find a value of {{val|299710|22|u=km/s}}.<ref name="Essen1948"/><ref name="RosaDorsey">{{cite journal
|last=Rosa |first=EB |last2=Dorsey |first2=NE
|year=1907
|title=The Ratio of the Electromagnetic and Electrostatic Units|journal=[[Bulletin of the Bureau of Standards]]
|volume=3
|issue=6 |page=433
|doi=10.1103/PhysRevSeriesI.22.367
|bibcode = 1906PhRvI..22..367R }}</ref>


Two very short sections of line carry both AC and DC power circuits. One set of such towers is near the terminal of {{tsl|en|HVDC Volgograd-Donbass|}} on Volga Hydroelectric Power Station. The other are two towers south of Stenkullen, which carry one circuit of HVDC Konti-Skan and üne circuit of the three-phase AC line Stenkullen-Holmbakullen.
===Cavity resonance===
[[File:Waves in Box.svg|thumb|right|Electromagnetic [[standing waves]] in a cavity.|alt=A box with three waves in it; there are one and a half wavelength of the top wave, one of the middle one, and a half of the bottom one.]]


Towers carrying AC circuits and DC electrode lines exist in a section of the powerline between Adalph Static Inverter Plant and Brookston the pylons carry the electrode line of HVDC {{tsl|en|Square Butte (transmission line)||Square Butte}}.
Another way to measure the speed of light is to independently measure the frequency ''f'' and wavelength ''λ'' of an electromagnetic wave in vacuum. The value of ''c'' can then be found by using the relation ''c''&nbsp;=&nbsp;''fλ''. One option is to measure the resonance frequency of a [[cavity resonator]]. If the dimensions of the resonance cavity are also known, these can be used determine the wavelength of the wave. In 1946, [[Louis Essen]] and A.C. Gordon-Smith establish the frequency for a variety of [[normal mode]]s of microwaves of a [[microwave cavity]] of precisely known dimensions. The dimensions were established to an accuracy of about ±0.8&nbsp;μm using gauges calibrated by interferometry.<ref name="Essen1948"/> As the wavelength of the modes was known from the geometry of the cavity and from [[electromagnetic theory]], knowledge of the associated frequencies enabled a calculation of the speed of light.<ref name="Essen1948">{{cite journal
|last=Essen |first=L
|last2=Gordon-Smith |first2=AC
|year=1948
|title=The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator
|journal=[[Proceedings of the Royal Society of London A]]
|volume=194 |issue=1038 |pages=348–361
|doi=10.1098/rspa.1948.0085
|bibcode=1948RSPSA.194..348E
|jstor=98293
}}</ref><ref>
{{cite journal
|last=Essen |first=L
|year=1947
|title=Velocity of Electromagnetic Waves
|journal=Nature
|volume=159 |issue=4044 |pages=611–612
|doi=10.1038/159611a0
|bibcode=1947Natur.159..611E
}}</ref>


The electrode line of HVDC {{tsl|en|CU (Powerline)||CU}} at the converter station at Coal Creek Station uses on a short section the towers of two AC lines as support.
The Essen–Gordon-Smith result, {{val|299792|9|u=km/s}}, was substantially more precise than those found by optical techniques.<ref name="Essen1948" /> By 1950, repeated measurements by Essen established a result of {{val|299792.5|3.0|u=km/s}}.<ref name="Essen1950">
{{cite journal
|last=Essen |first=L
|year=1950
|title=The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator
|journal=[[Proceedings of the Royal Society of London A]]
|volume=204 |issue=1077 |pages=260–277
|doi=10.1098/rspa.1950.0172
|bibcode=1950RSPSA.204..260E
|jstor=98433
}}</ref>


The overhead section of the {{tsl|en|electrode line|}} of {{tsl|en|Pacific DC Intertie|}} from Sylmar Converter Station to the grounding electrode in the Pacific Ocean near {{tsl|en|Will Rogers State Beach|}} is also installed on AC pylons. It runs from Sylmar East Converter Station to Southern California Edison Malibu Substation, where the overhead line section ends.
A household demonstration of this technique is possible, using a [[microwave oven]] and food such as marshmallows or margarine: if the turntable is removed so that the food does not move, it will cook the fastest at the [[antinode]]s (the points at which the wave amplitude is the greatest), where it will begin to melt. The distance between two such spots is half the wavelength of the microwaves; by measuring this distance and multiplying the wavelength by the microwave frequency (usually displayed on the back of the oven, typically 2450&nbsp;MHz), the value of ''c'' can be calculated, "often with less than 5% error".<ref>
{{cite journal
| last = Stauffer | first = RH
| year = 1997 | month = April
| title = Finding the Speed of Light with Marshmallows
| journal = [[The Physics Teacher]]
| volume = 35
| page = 231
| publisher = American Association of Physics Teachers
| url = http://www.physics.umd.edu/icpe/newsletters/n34/marshmal.htm
| accessdate = 2010-02-15
|bibcode = 1997PhTea..35..231S |doi = 10.1119/1.2344657
| issue = 4 }}</ref><ref>{{cite web
| url =http://www.bbc.co.uk/norfolk/features/ba_festival/bafestival_speedoflight_experiment_feature.shtml
| title = BBC Look East at the speed of light
| work = BBC Norfolk website
|publisher=BBC
| accessdate = 2010-02-15
}}</ref>


In Germany, Austria and Switzerland some transmission towers carry both public AC grid circuits and railway traction power in order to better use rights of way.
===Interferometry===
[[File:Interferometer sol.svg|thumb|upright=1.4|An interferometric determination of length. Left: [[constructive interference]]; Right: [[destructive interference]].|alt=Schematic of the working of a Michelson interferometer.]]
[[Interferometry]] is another method to find the wavelength of electromagnetic radiation for determining the speed of light.<ref name=Vaughan>
A detailed discussion of the interferometer and its use for determining the speed of light can be found in {{Cite book
|last=Vaughan |first=JM
|year=1989
|title=The Fabry-Perot interferometer
|url=http://books.google.com/?id=mMLuISueDKYC&printsec=frontcover#PPA47,M1
|page=47, pp.&nbsp;384–391
|publisher=CRC Press
|isbn=0-85274-138-3
}}</ref> A [[Coherence (physics)|coherent]] beam of light (e.g. from a [[laser]]), with a known frequency (''f''), is split to follow two paths and then recombined. By adjusting the path length while observing the [[interference (wave propagation)|interference pattern]] and carefully measuring the change in path length, the wavelength of the light (''λ'') can be determined. The speed of light is then calculated using the equation&nbsp;''c''&nbsp;=&nbsp;''λf''.


== Tower designs ==
Before the advent of laser technology, coherent [[radiowave|radio]] sources were used for interferometry measurements of the speed of light.<ref name=Froome1858>
{{cite journal
|doi=10.1098/rspa.1958.0172
|title=A New Determination of the Free-Space Velocity of Electromagnetic Waves
|first=KD
|last=Froome
|journal=Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences,
|volume=247
|year=1958
|pages=109–122
|issue=1248
|publisher=The Royal Society
|bibcode = 1958RSPSA.247..109F
|jstor=100591 }}</ref> However interferometric determination of wavelength becomes less precise with wavelength and the experiments were thus limited in precision by the long wavelength (~0.4&nbsp;cm) of the radiowaves. The precision can be improved by using light with a shorter wavelength, but then it becomes difficult to directly measure the frequency of the light. One way around this problem is to start with a low frequency signal of which the frequency can be precisely measured, and from this signal progressively synthesize higher frequency signals whose frequency can then be linked to the original signal. A laser can then be locked to the frequency, and its wavelength can be determined using interferometry.<ref name="NIST_pub">
{{Cite book
|title=A Century of Excellence in Measurements, Standards, and Technology
|editor-last=Lide |editor-first=DR
|contribution=Speed of Light from Direct Frequency and Wavelength Measurements
|last=Sullivan |first=DB
|year=2001
|pages=191–193
|publisher=CRC Press
|isbn=0-8493-1247-7
|url=http://nvl.nist.gov/pub/nistpubs/sp958-lide/191-193.pdf
}}</ref> This technique was due to a group at the National Bureau of Standards (NBS) (which later became [[National Institute of Standards and Technology|NIST]]). They used it in 1972 to measure the speed of light in vacuum with a [[Measurement uncertainty|fractional uncertainty]] of {{val|3.5|e=-9}}.<ref name="NIST_pub"/><ref name="NIST heterodyne">
{{cite journal
|last1=Evenson |first1=KM |coauthors=''et al.''
|year=1972
|title=Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser
|journal=Physical Review Letters
|volume=29
|issue=19 |pages=1346–49
|doi=10.1103/PhysRevLett.29.1346
|bibcode=1972PhRvL..29.1346E
}}</ref>


==History==
=== Shape ===
[[File:Guyed Delta Transmission Tower.jpg|thumb|Guyed "Delta" transmission tower (a combination of guyed "V" and "Y") in [[内华达州]].]]
{| class="infobox wikitable" style="width:40%; margin:0 0 0.5em 1em; text-align:left;"
|+History&nbsp;of&nbsp;measurements&nbsp;of&nbsp;''c'' (in km/s)
|-
|1675||[[Ole&nbsp;Rømer|Rømer]]&nbsp;and&nbsp;[[Christiaan&nbsp;Huygens|Huygens]], moons&nbsp;of&nbsp;Jupiter||{{val|220000}}<ref name=roemer/><ref name="Huygens 1690 8–9"/>
|-
|1729||[[James&nbsp;Bradley]], aberration&nbsp;of&nbsp;light||{{val|301000}}<ref name=How/>
|-
|1849||[[Hippolyte&nbsp;Fizeau]], toothed&nbsp;wheel||{{val|315000}}<ref name=How/>
|-
|1862||[[Léon&nbsp;Foucault]], rotating&nbsp;mirror||{{val|298000|500}}<ref name=How/>
|-
|1907||Rosa&nbsp;and&nbsp;Dorsey, <abbr title="electromagnetic">EM</abbr>&nbsp;constants||{{val|299710|30}}<ref name="Essen1948"/><ref name="RosaDorsey"/>
|-
|1926||[[Albert&nbsp;Michelson]], rotating&nbsp;mirror||{{val|299796|4}}<ref>{{cite doi|10.1086/143021}}</ref>
|-
|1950||{{nowrap|Essen and Gordon-Smith}}, cavity&nbsp;resonator||{{val|299792.5|3.0}}<ref name="Essen1950"/>
|-
|1958||K.D.&nbsp;Froome, radio&nbsp;interferometry||{{val|299792.50|0.10}}<ref name="Froome1858"/>
|-
|1972||Evenson&nbsp;''et&nbsp;al.'', laser&nbsp;interferometry||{{val|299792.4562|0.0011}}<ref name="NIST heterodyne"/>
|-
|1983||17th&nbsp;CGPM, definition&nbsp;of&nbsp;the&nbsp;metre||{{val|299792.458}}&nbsp;(exact)<ref name=Resolution_1/>
|}
Until the [[early modern period]], it was not known whether light travelled instantaneously or at a very fast finite speed. The first extant recorded examination of this subject was in [[ancient Greece]]. The ancient Greeks, Muslim scholars and classical European scientists long debated this until Rømer provided the first calculation of the speed of light. Einstein's Theory of Special Relativity concluded that the speed of light is constant regardless of one's frame of reference. Since then, scientists have provided increasingly accurate measurements.


Different shapes of transmission towers are typical for different countries. The shape also depends on voltage and number of circuits.
===Early history===
[[Empedocles]] was the first to claim that light has a finite speed.<ref>
{{Cite book
|last=Sarton |first=G
|year=1993
|title=Ancient science through the golden age of Greece
|url=http://books.google.com/?id=VcoGIKlHuZcC&pg=PA248
|page=248
|publisher=[[Courier Dover]]
|isbn=0-486-27495-0
}}</ref> He maintained that light was something in motion, and therefore must take some time to travel. [[Aristotle]] argued, to the contrary, that "light is due to the presence of something, but it is not a movement".<ref name=Statistics>
{{cite journal
|last=MacKay |first=RH |last2=Oldford |first2=RW
|year=2000
|title=Scientific Method, Statistical Method and the Speed of Light
|url=http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/
|journal=[[Statistical Science (journal)|Statistical Science]]
|volume=15 |issue=3 |pages=254–78
|doi=10.1214/ss/1009212817
}} (click on "Historical background" in the table of contents)</ref> [[Euclid]] and [[Ptolemy]] advanced the [[Emission theory (vision)|emission theory]] of vision, where light is emitted from the eye, thus enabling sight. Based on that theory, [[Heron of Alexandria]] argued that the speed of light must be [[infinite]] because distant objects such as stars appear immediately upon opening the eyes.


====One circuit====
[[Early Islamic philosophy|Early Islamic philosophers]] initially agreed with the [[Aristotelian physics|Aristotelian view]] that light had no speed of travel. In 1021, [[Alhazen]] (Ibn al-Haytham) published the ''[[Book of Optics]]'', in which he presented a series of arguments dismissing the emission theory in favour of the now accepted intromission theory of [[Visual perception|vision]], in which light moves from an object into the eye.<ref>{{cite doi|10.1177/107385849900500108}}</ref>{{verify source|date=January 2012}} This led Alhazen to propose that light must have a finite speed,<ref name=Statistics/><ref name=Hamarneh>
Delta pylons are the most common design for single circuit lines, because of their stability. They have a V-shaped body with a horizontal arm on the top, which forms an inverted [[Δ|Delta]]. Larger Delta towers usually use two guard cables.
{{cite journal
|last=Hamarneh |first=S
|year=1972
|title=Review: Hakim Mohammed Said, ''Ibn al-Haitham''
|journal=[[Isis (journal)|Isis]]
|volume=63 |issue=1 |page=119
|doi=10.1086/350861
}}</ref><ref name=Lester>
{{Cite book
|last=Lester |first=PM
|year=2005
|title=Visual Communication: Images With Messages
|pages=10–11
|publisher=[[Thomson Wadsworth]]
|isbn=0-534-63720-5
}}</ref> and that the speed of light is variable, decreasing in denser bodies.<ref name=Lester/><ref>
{{cite web
|first1=JJ
|last1=O'Connor
|authorlink1=John J. O'Connor (mathematician)
|first2=EF
|last2=Robertson
|authorlink2=Edmund F. Robertson
|url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Al-Haytham.html
|title=Abu Ali al-Hasan ibn al-Haytham
|work=[[MacTutor History of Mathematics archive]]
|publisher=[[University of St Andrews]]
|accessdate=2010-01-12
}}</ref> He argued that light is substantial matter, the propagation of which requires time, even if this is hidden from our senses.<ref>
{{cite conference
|last=Lauginie |first=P
|year=2005
|title=Measuring: Why? How? What?
|url=http://www.ihpst2005.leeds.ac.uk/papers/Lauginie.pdf
|booktitle=Proceedings of the 8th International History, Philosophy, Sociology & Science Teaching Conference
|accessdate=2008-07-18
}}</ref> Also in the 11th century, [[Abū Rayhān al-Bīrūnī]] agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.<ref>
{{cite web
|first1=JJ
|last1=O'Connor
|first2=EF
|last2=Robertson
|url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Al-Biruni.html
|title=Abu han Muhammad ibn Ahmad al-Biruni
|work=MacTutor History of Mathematics archive
|publisher=University of St Andrews
|accessdate=2010-01-12
}}</ref>


Portal pylons are widely used in Ireland, Scandinavia and Canada. They stand on two legs with one cross arm, which gives them a H-shape. Up to 110 kV they often were made from wood, but higher voltage lines use steel pylons.
In the 13th century, [[Roger Bacon]] argued that the speed of light in air was not infinite, using philosophical arguments backed by the writing of Alhazen and Aristotle.<ref name=Lindberg>
{{Cite book
|last=Lindberg |first=DC
|year=1996
|title=Roger Bacon and the origins of Perspectiva in the Middle Ages: a critical edition and English translation of Bacon's Perspectiva, with introduction and notes
|url=http://books.google.com/?id=jSPHMKbjYkQC&pg=PA143
|page=143
|isbn=0-19-823992-0
|publisher=[[Oxford University Press]]
}}</ref><ref>
{{Cite book
|last=Lindberg |first=DC
|year=1974
|chapter=Late Thirteenth-Century Synthesis in Optics
|editor=Edward Grant
|title=A source book in medieval science
|url=http://books.google.com/?id=fAPN_3w4hAUC&pg=RA1-PA395&dq=roger-bacon+speed-of-light&q=roger-bacon%20speed-of-light
|page=396
|publisher=[[Harvard University Press]]
|isbn=978-0-674-82360-0
}}</ref> In the 1270s, [[Witelo]] considered the possibility of light travelling at infinite speed in vacuum, but slowing down in denser bodies.<ref name=Marshall>
{{cite journal
|last=Marshall |first=P
|year=1981
|title=Nicole Oresme on the Nature, Reflection, and Speed of Light
|journal=[[Isis (journal)|Isis]]
|volume=72 |issue=3 |pages=357–74 [367–74]
|doi=10.1086/352787
}}</ref>


Smaller single circuit pylons may have two small cross arms on one side and one on the other.
In the early 17th century, [[Johannes Kepler]] believed that the speed of light was infinite, since empty space presents no obstacle to it. [[René Descartes]] argued that if the speed of light were finite, the Sun, Earth, and Moon would be noticeably out of alignment during a [[lunar eclipse]]. Since such misalignment had not been observed, Descartes concluded the speed of light was infinite. Descartes speculated that if the speed of light were found to be finite, his whole system of philosophy might be demolished.<ref name=Statistics />


===First measurement attempts===
====Two circuits====
One level pylons only have one cross arm carrying 3 cables on each side. Sometimes they have an additional cross arm for the protection cables. They are frequently used close to airports due to their reduced height.


[[File:Strelasund-160324-103a.jpg|thumb|Typical T-shaped 110 kV tower from the former [[東德]].]]
In 1629, [[Isaac Beeckman]] proposed an experiment in which a person observes the flash of a cannon reflecting off a mirror about one mile (1.6&nbsp;km) away. In 1638, [[Galileo Galilei]] proposed an experiment, with an apparent claim to having performed it some years earlier, to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. He was unable to distinguish whether light travel was instantaneous or not, but concluded that if it were not, it must nevertheless be extraordinarily rapid.<ref name=boyer>
{{cite journal
|last=Boyer |first=CB
|year=1941
|title=Early Estimates of the Velocity of Light
|journal=[[Isis (journal)|Isis]]
|volume=33 |issue=1 |page=24
|doi=10.1086/358523
|ref=boyer-1941
}}</ref><ref name=2newsciences>
{{Cite book
|last=Galilei |first=G
|year=1954 |origyear=1638
|title=Dialogues Concerning Two New Sciences
|url=http://oll.libertyfund.org/index.php?option=com_staticxt&staticfile=show.php%3Ftitle=753&layout=html#a_2288356
|page=43
|others=Crew, H; de Salvio A (trans.)
|publisher=[[Dover Publications]]
|isbn=0-486-60099-8
|ref=Reference-Galileo-1954
}}</ref> Galileo's experiment was carried out by the [[Accademia del Cimento]] of Florence, Italy, in 1667, with the lanterns separated by about one&nbsp;mile, but no delay was observed. The actual delay in this experiment would have been about 11 [[microsecond]]s.


Danube pylons or ''Donaumasten'' got their name from a line built in 1927 next to the [[多瑙河]]. They are the most common design in central European countries like Germany or Poland. They have two cross arms, the upper arm carries one and the lower arm carries two cables on each side. Sometimes they have an additional cross arm for the protection cables.
[[File:Illustration from 1676 article on Ole Rømer's measurement of the speed of light.jpg|thumb|left|upright=0.8|Rømer's observations of the occultations of Io from Earth|alt=A diagram of a planet's orbit around the Sun and of a moon's orbit around another planet. The shadow of the latter planet is shaded.]]
The first quantitative estimate of the speed of light was made in 1676 by Rømer (see [[Rømer's determination of the speed of light]]).<ref name="cohen"/><ref name="roemer"/> From the observation that the periods of Jupiter's innermost moon [[Io (moon)|Io]] appeared to be shorter when the Earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and estimated that it takes light 22 minutes to cross the diameter of Earth's orbit. [[Christiaan Huygens]] combined this estimate with an estimate for the diameter of the Earth's orbit to obtain an estimate of speed of light of {{val|220000|u=km/s}}, 26% lower than the actual value.<ref name="Huygens 1690 8–9">{{Cite book
|last=Huygens |first=C
|year=1690
|title=Traitée de la Lumière |language=French
|url=http://books.google.com/?id=No8PAAAAQAAJ&pg=PA9
|pages=8–9
|publisher=[[Pierre van der Aa]]
}}</ref>


Ton shaped towers are the most common design, they have 3 horizontal levels with one cable very close to the pylon on each side. In the United Kingdom the second level is often (but not always) wider than the other ones while in the United States all cross arms have the same width.
In his 1704 book ''[[Opticks]]'', [[Isaac Newton]] reported Rømer's calculations of the finite speed of light and gave a value of "seven or eight minutes" for the time taken for light to travel from the Sun to the Earth (the modern value is 8&nbsp;minutes 19&nbsp;seconds).<ref>
{{Cite book
|last=Newton |first=I
|year=1704
|contribution=Prop. XI
|title=Optiks
|url=http://gallica.bnf.fr/ark:/12148/bpt6k3362k.image.f235.vignettesnaviguer
}} The text of Prop.&nbsp;XI is identical between the first (1704) and second (1719) editions.</ref> Newton queried whether Rømer's eclipse shadows were coloured; hearing that they were not, he concluded the different colours travelled at the same speed. In 1729, [[James Bradley]] discovered the [[aberration of light]].<ref name="Bradley1729"/> From this effect he determined that light must travel 10,210 times faster than the Earth in its orbit (the modern figure is 10,066 times faster) or, equivalently, that it would take light 8&nbsp;minutes 12&nbsp;seconds to travel from the Sun to the Earth.<ref name="Bradley1729"/>


[[File:Electricity Wire Annotated.jpg|thumb|A close up of the wires attached to the pylon, showing the various parts annotated.]]
===Connections with electromagnetism===
{{See also|History of electromagnetic theory|History of special relativity}}
In the 19th century [[Hippolyte Fizeau]] developed a method to determine the speed of light based on time-of-flight measurements on Earth and reported a value of {{val|315000|u=km/s}}. His method was improved upon by [[Léon Foucault]] who obtained a value of {{val|298000|u=km/s}} in 1862.<ref name="How"/> In the year 1856, [[Wilhelm Eduard Weber]] and [[Rudolf Kohlrausch]] measured the ratio of the electromagnetic and electrostatic units of charge, 1/√''ε''<sub>0</sub>''μ''<sub>0</sub>, by discharging a [[Leyden jar]], and found that its numerical value was very close to the speed of light as measured directly by Fizeau. The following year [[Gustav Kirchhoff]] calculated that an electric signal in a [[electrical resistance|resistanceless]] wire travels along the wire at this speed.<ref>{{cite journal
|last1=Graneau |first1=P
|last2=Assis |first2=AKT
|title=Kirchhoff on the motion of electricity in conductors
|journal=[[Apeiron (physics journal)|Apeiron]]
|volume=19
|year=1994
|pages=19–25
|url=http://www.physics.princeton.edu/~mcdonald/examples/EM/kirchhoff_apc_102_529_57_english.pdf
|accessdate=2010-10-21
}}</ref> In the early 1860s, Maxwell showed that according to the theory of electromagnetism which he was working on, that electromagnetic waves propagate in empty space<ref>{{cite book
|title=College physics: reasoning and relationships
|first1=Nicholas J.
|last1=Giordano
|publisher=Cengage Learning
|year=2009
|isbn=0-534-42471-6
|page=787
|url=http://books.google.com/books?id=BwistUlpZ7cC}}, [http://books.google.com/books?id=BwistUlpZ7cC&pg=PA787 Extract of page 787]
</ref><ref>{{cite book
|title=The riddle of gravitation
|first1=Peter Gabriel
|last1=Bergmann
|publisher=Courier Dover Publications
|year=1992
|isbn=0-486-27378-4
|page=17
|url=http://books.google.com/books?id=WYxkrwMidp0C}}, [http://books.google.com/books?id=WYxkrwMidp0C&pg=PA17 Extract of page 17]
</ref><ref>{{cite book
|title=The equations: icons of knowledge
|first1=Sander
|last1=Bais
|publisher=Harvard University Press
|year=2005
|isbn=0-674-01967-9
|page=40
|url=http://books.google.com/books?id=jKbVuMSlJPoC}}, [http://books.google.com/books?id=jKbVuMSlJPoC&pg=PA40 Extract of page 40]
</ref> at a speed equal to the above Weber/Kohrausch ratio, and drawing attention to the numerical proximity of this value to the speed of light as measured by Fizeau, he proposed that light is in fact an electromagnetic wave.<ref name=maxwellbio>
{{cite web
|last1=O'Connor |first1=JJ |last2=Robertson |first2=EF
|date=November 1997
|title= James Clerk Maxwell
|url= http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Maxwell.html
|publisher=School of Mathematics and Statistics, [[University of St Andrews]]
|accessdate=2010-10-13
}}</ref>


==="Luminiferous aether"===
====Four circuits====
Christmas-tree-shaped towers for 4 or even 6 circuits are common in Germany and have 3 cross arms where the highest arm has each one cable, the second has two cables and the third has three cables on each side. The cables on the third arm usually carry circuits for lower high voltage.
[[File:Einstein en Lorentz.jpg|thumb|150px|Hendrik Lorentz with Albert Einstein.]]
It was thought at the time that empty space was filled with a background medium called the [[luminiferous aether]] in which the electromagnetic field existed. Some physicists thought that this aether acted as a [[preferred frame]] of reference for the propagation of light and therefore it should be possible to measure the motion of the Earth with respect to this medium, by measuring the isotropy of the speed of light. Beginning in the 1880s several experiments were performed to try to detect this motion, the most famous of which is [[Michelson–Morley experiment|the experiment]] performed by [[Albert Michelson]] and [[Edward Morley]] in 1887.<ref>
{{Cite journal
|last1=Michelson |first1=AA |last2=Morley |first2=EW
|year=1887
|title=[[s:On the Relative Motion of the Earth and the Luminiferous Ether|On the Relative Motion of the Earth and the Luminiferous Ether]]
|journal=[[American Journal of Science]]
|volume=34 |pages=333–345
|doi=
}}</ref> The detected motion was always less than the observational error. Modern experiments indicate that the two-way speed of light is [[isotropic]] (the same in every direction) to within 6 nanometres per second.<ref>
{{Cite book
| last = French | first = AP
| year = 1983
| title = Special relativity
| pages = 51–57 | publisher = Van Nostrand Reinhold
| isbn = 0-442-30782-9
}}</ref>
Because of this experiment [[Hendrik Lorentz]] proposed that the motion of the apparatus through the aether may cause the apparatus to [[Lorentz contraction|contract]] along its length in the direction of motion, and he further assumed, that the time variable for moving systems must also be changed accordingly ("local time"), which led to the formulation of the [[Lorentz transformation]]. Based on [[Lorentz ether theory|Lorentz's aether theory]], [[Henri Poincaré]] (1900) showed that this local time (to first order in v/c) is indicated by clocks moving in the aether, which are synchronized under the assumption of constant light speed. In 1904, he speculated that the speed of light could be a limiting velocity in dynamics, provided that the assumptions of Lorentz's theory are all confirmed. In 1905, Poincaré brought Lorentz's aether theory into full observational agreement with the [[principle of relativity]].<ref>
{{Cite book
|last=Darrigol |first=O
|year=2000
|title= Electrodynamics from Ampére to Einstein
|publisher=Clarendon Press
|isbn=0-19-850594-9}}</ref><ref>{{Cite book
|last=Galison |first=P
|authorlink=Peter Galison
|year=2003
|title= Einstein's Clocks, Poincaré's Maps: Empires of Time
|publisher=W.W. Norton
|isbn=0-393-32604-7}}</ref>


===Special relativity===
=== Support structures ===
[[File:58730_Fr%C3%B6ndenberg,_Germany_-_panoramio_-_Foto_Fitti_(24).jpg|thumb|Danube pole for 110 kV in Germany, built in the 1930s]]
In 1905 Einstein postulated from the outset that the speed of light in vacuum, measured by a non-accelerating observer, is independent of the motion of the source or observer. Using this and the principle of relativity as a basis he derived the [[special theory of relativity]], in which the speed of light in vacuum ''c'' featured as a fundamental constant, also appearing in contexts unrelated to light. This made the concept of the stationary aether (to which Lorentz and Poincaré still adhered) useless and revolutionized the concepts of space and time.<ref>{{Cite book
|last=Miller |first=AI
|year=1981
|title= Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911)
|publisher=Addison–Wesley
|isbn=0-201-04679-2}}</ref><ref>
{{Cite book
|last=Pais |first=A
|authorlink=Abraham Pais
|year=1982
|title= Subtle is the Lord: The Science and the Life of Albert Einstein
|publisher=Oxford University Press
|isbn=0-19-520438-7}}</ref>


Towers may be self-supporting and capable of resisting all forces due to conductor loads, unbalanced conductors, wind and ice in any direction. Such towers often have approximately square bases and usually four points of contact with the ground.
===Increased accuracy of ''c'' and redefinition of the metre===
{{See also|History of the metre}}
In the second half of the 20th century much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques. In 1972, using the latter method and the [[history of the metre#Krypton standard|1960 definition of the metre]] in terms of a particular spectral line of krypton-86, a group at [[National Institute of Standards and Technology|NBS]] in [[Boulder, Colorado]] determined the speed of light in vacuum to be ''c''&nbsp;=&nbsp;{{val|299792456.2|1.1|u=m/s}}. This was 100 times less [[Measurement uncertainty|uncertain]] than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre.{{#tag:ref|Since 1960 the metre was defined as: "The metre is the length equal to {{val|1650763.73}} wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p<sub>10</sub> and 5d<sub><sub>5</sub></sub> of the krypton 86 atom."<ref name="11thCGPM">
{{cite web
|year=1967
|title=Resolution 6 of the 15th CGPM
|url=http://www.bipm.org/en/CGPM/db/11/6/
|publisher=[[International Bureau of Weights and Measures|BIPM]]
|accessdate=2010-10-13
}}</ref> It was later discovered that this spectral line was not symmetric, which put a limit on the precision with which the definition could be realized in interferometry experiments.<ref>{{cite doi|10.1063/1.1654608}}
</ref>|group="Note"}}<ref name="NIST heterodyne"/> Since similar experiments found comparable results for ''c'', the 15th&nbsp;[[Conférence Générale des Poids et Mesures]] (CGPM) in 1975 recommended using the value {{val|299792458|u=m/s}} for the speed of light.<ref name="15thCGPM">
{{cite web
|year=1975
|title=Resolution 2 of the 15th CGPM
|url=http://www.bipm.org/en/CGPM/db/15/2/
|publisher=BIPM
|accessdate=2009-09-09
}}</ref>


A semi-flexible tower is designed so that it can use overhead grounding wires to transfer mechanical load to adjacent structures, if a phase conductor breaks and the structure is subject to unbalanced loads. This type is useful at extra-high voltages, where phase conductors are bundled (two or more wires per phase). It is unlikely for all of them to break at once, barring a catastrophic crash or storm.
In 1983 the 17th&nbsp;CGPM redefined the metre thus, "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299&nbsp;792&nbsp;458 of a second."<ref name=Resolution_1>
{{cite web
|year=1983
|title=Resolution 1 of the 17th CGPM
|url=http://www.bipm.org/en/CGPM/db/17/1/
|publisher=BIPM
|accessdate=2009-08-23
}}</ref> As a result of this definition, the value of the speed of light in vacuum is exactly {{val|299792458|u=m/s}}<ref name="Wheeler"/><ref name=timeline>
{{cite web
|last=Penzes |first=WB
|year=2009
|title=Time Line for the Definition of the Meter
|url=http://www.nist.gov/pml/div683/upload/museum-timeline.pdf
|publisher=[[National Institute of Standards and Technology|NIST]]
|accessdate=2010-01-11
}}</ref> and has become a defined constant in the SI system of units.<ref name="Jespersen"/> Improved experimental techniques do not affect the value of the speed of light in SI units, but instead allow for a more precise realization of the definition of the metre.<ref name=Adams>
{{Cite book
|last=Adams |first=S
|year=1997
|title=Relativity: An Introduction to Space-Time Physics
|url=http://books.google.com/?id=1RV0AysEN4oC&pg=PA140
|page=140
|publisher=CRC Press
|isbn=0-7484-0621-2
|quote=One peculiar consequence of this system of definitions is that any future refinement in our ability to measure&nbsp;''c'' will not change the speed of light (which is a defined number), but will change the length of the meter!
}}</ref><ref name=W_Rindler>
{{Cite book
|last=Rindler |first=W
|year=2006
|title=Relativity: Special, General, and Cosmological
|url=http://books.google.com/?id=MuuaG5HXOGEC&pg=PT41
|page=41
|edition=2nd
|publisher=[[Oxford University Press]]
|isbn=0-19-856731-6
|quote=Note that [...] improvements in experimental accuracy will modify the meter relative to atomic wavelengths, but not the value of the speed of light!
}}</ref>


A {{tsl|en|guyed mast|}} has a very small footprint and relies on guy wires in tension to support the structure and any unbalanced tension load from the conductors. A guyed tower can be made in a V shape, which saves weight and cost.<ref name=BEATY78 />
==See also==
*[[Light-second]]


==Notes==
=== Materials ===
{{reflist|group="Note"|3}}


==== Tubular steel ====
==References==
{{Reflist|3}}


[[File:New and old electricity pylons.jpg|thumb|upright|Steel tube tower next to older lattice tower near [[沃加沃加]], Australia]]
==Further reading==
===Historical references===
{{Refbegin}}
*{{Cite journal
|first=O |last=Rømer |author-link=Ole Rømer
|year=1676
|title=Démonstration touchant le mouvement de la lumière trouvé par M. Römer de l'Academie Royale des Sciences
|url=http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Roemer-1677/Roemer-1677.html
|journal=[[Journal des sçavans]]
|pages=223–36
|language=French
|archiveurl=http://web.archive.org/web/20070729214326/http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Roemer-1677/Roemer-1677.html
|archivedate=2007-07-29
}}
** Translated as {{cite journal
|year=1677
|title=A Demonstration concerning the Motion of Light
|url=http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Roemer-1677/Roemer-1677.html
|journal=[[Philosophical Transactions of the Royal Society]]
|issue=136 |pages=893–4
|archiveurl = http://web.archive.org/web/20070729214326/http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Roemer-1677/Roemer-1677.html
|archivedate = 2007-07-29
}}
*{{Cite journal
|first=E |last=Halley |author-link=Edmond Halley
|year=1694
|title=Monsieur Cassini, his New and Exact Tables for the Eclipses of the First Satellite of Jupiter, reduced to the Julian Stile and Meridian of London
|journal=[[Philosophical Transactions of the Royal Society]]
|volume=18 |issue=214 |pages=237–56
|doi=10.1098/rstl.1694.0048
}}
*{{Cite journal
|first=HL |last=Fizeau |author-link=Hippolyte Fizeau
|year=1849
|title=Sur une expérience relative à la vitesse de propagation de la lumière
|url=http://web.archive.org/web/20110613224002/http://www.academie-sciences.fr/membres/in_memoriam/Fizeau/Fizeau_pdf/CR1849_p90.pdf
|journal=[[Comptes rendus de l'Académie des sciences]]
|volume=29 |pages=90–92, 132
|language=French
}}
*{{Cite journal
|first=JL |last=Foucault |author-link=Léon Foucault
|year=1862
|title=Détermination expérimentale de la vitesse de la lumière: parallaxe du Soleil
|url=http://books.google.ca/books?id=yYIIAAAAMAAJ&pg=PA216&lpg=PA216&dq
|journal=[[Comptes rendus de l'Académie des sciences]]
|volume=55 |pages=501–503, 792–796
|language=French
}}
*{{Cite journal
|first=AA |last=Michelson |author-link=Albert Abraham Michelson
|year=1878
|title=Experimental Determination of the Velocity of Light
|url=http://www.gutenberg.org/ebooks/11753
|journal=[[Proceedings of the American Association of Advanced Science]]
|volume=27 |pages=71–77
}}
*{{Cite journal
|first1=AA |last1=Michelson
|first2=FG |last2=Pease |author2-link=Francis Gladheim Pease
|first3=F |last3=Pearson |author3-link=F. Pearson
|title=Measurement of the Velocity of Light in a Partial Vacuum
|journal=[[Astrophysical Journal]]
|volume=82 |pages=26–61 |year=1935
|doi=10.1086/143655 |bibcode=1935ApJ....82...26M
}}
*{{Cite journal
|first=S |last=Newcomb |author-link=Simon Newcomb
|year=1886
|title=The Velocity of Light
|journal=[[Nature (journal)|Nature]]
|volume=34
|issue=863 |pages=29–32
|doi=10.1038/034029c0
|bibcode = 1886Natur..34...29. }}
*{{Cite journal
|first=J |last=Perrotin |author-link=Henri Joseph Anastase Perrotin
|year=1900
|title=Sur la vitesse de la lumière
|journal=[[Comptes rendus de l'Académie des sciences]]
|volume=131 |pages=731–4
|language=French
}}
{{Refend}}


Poles made of tubular [[钢]] generally are assembled at the factory and placed on the right-of-way afterward. Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer the use of monopolar steel or concrete towers over lattice steel for new power lines and tower replacements. {{Citation needed|date=November 2007}}
===Modern references===


{{tsl|en|Energy in Germany||In Germany}} steel tube pylons are also established predominantly for medium voltage lines, in addition, for high voltage transmission lines or two electric circuits for operating voltages by up to 110 kV. Steel tube pylons are also frequently used for 380 kV lines {{tsl|en|Energy in France||in France}}, and for 500 kV lines {{tsl|en|Energy in the United States||in the United States}}.
{{Refbegin}}
*{{Cite book
|first=L |last=Brillouin |author-link=Léon Brillouin
|year=1960
|title=Wave propagation and group velocity
|publisher=[[Academic Press]]
|isbn=
}}
*{{Cite book
|first=JD |last=Jackson |author-link=J. D. Jackson
|year=1975
|title=Classical Electrodynamics
|edition=2nd
|publisher=[[John Wiley & Sons]]
|isbn=0-471-30932-X
}}
*{{Cite book
|first=G |last=Keiser
|year=2000
|title=Optical Fiber Communications
|page=32 |edition=3rd
|publisher=[[McGraw-Hill]]
|isbn=0-07-232101-6
}}
*{{Cite book
|last=Ng |first=YJ
|year=2004
|chapter=Quantum Foam and Quantum Gravity Phenomenology
|url=http://books.google.com/?id=RntpN7OesBsC
|editor=Amelino-Camelia, G; Kowalski-Glikman, J
|title=Planck Scale Effects in Astrophysics and Cosmology
|pages=321''ff''
|publisher=[[Springer (publisher)|Springer]]
|isbn=3-540-25263-0
}}
*{{Cite book
|last=Helmcke |first=J |last2=Riehle |first2=F
|year=2001
|chapter=Physics behind the definition of the meter
|url=http://books.google.com/?id=WE22Fez60EcC&pg=PA453
|editor=Quinn, TJ; Leschiutta, S; Tavella, P
|title=Recent advances in metrology and fundamental constants
|page=453
|publisher=[[IOS Press]]
|isbn=1-58603-167-8
}}
*{{cite arxiv
|last=Duff |first=MJ |author-link=Michael James Duff
|year=2004
|title=Comment on time-variation of fundamental constants
|class=hep-th
|eprint=hep-th/0208093
}}
{{Refend}}


==External links==
==== Lattice ====
*[http://physics.nist.gov/cgi-bin/cuu/Value?c Speed of light in vacuum] (National Institute of Standards and Technology, NIST)
*[http://www.bipm.org/en/si/si_brochure/chapter2/2-1/metre.html Definition of the metre] (International Bureau of Weights and Measures, BIPM)
*[http://www.itl.nist.gov/div898/bayesian/datagall/michelso.htm Data Gallery: Michelson Speed of Light (Univariate Location Estimation)] (download data gathered by [[Albert Abraham Michelson|A.A. Michelson]])
*[http://gregegan.customer.netspace.net.au/APPLETS/20/20.html Subluminal] (Java applet demonstrating group velocity information limits)
*[http://www.mathpages.com/rr/s3-03/3-03.htm De Mora Luminis] at MathPages
*[http://www.ertin.com/sloan_on_speed_of_light.html Light discussion on adding velocities]
*[http://www.colorado.edu/physics/2000/waves_particles/lightspeed-1.html Speed of Light] (University of Colorado Department of Physics)
*[http://sixtysymbols.com/videos/light.htm c: Speed of Light] (Sixty Symbols, University of Nottingham Department of Physics [video])
*[http://math.ucr.edu/home/baez/physics/ Usenet Physics FAQ]
*[http://njsas.org/projects/speed_of_light/fizeau/ The Fizeau "Rapidly Rotating Toothed Wheel" Method]
<!-- en-GB-oed, -ize -->
{{extreme motion}}


{{See also|Lattice tower}}
{{DEFAULTSORT:Speed Of Light}}
[[Category:Fundamental constants]]
[[Category:Fundamental physics concepts]]
[[Category:Light]]
[[Category:Special relativity]]
[[Category:Units of velocity]]


A lattice tower is a framework construction made of steel or aluminium sections. Lattice towers are used for [[Overhead power line|power lines]] of all voltages, and are the most common type for high-voltage transmission lines. Lattice towers are usually made of galvanized steel. Aluminium is used for reduced weight, such as in mountainous areas where structures are placed by helicopter. Aluminium is also used in environments that would be corrosive to steel. The extra material cost of aluminium towers will be offset by lower installation cost. Design of aluminium lattice towers is similar to that for steel, but must take into account aluminium's lower [[杨氏模量]].
{{Link FA|de}}
{{Link FA|es}}
{{Link FA|sk}}


A lattice tower is usually assembled at the location where it is to be erected. This makes very tall towers possible, up to {{convert|100|m|ft|0|abbr=on}} (and in special cases even higher, as in the {{tsl|en|Elbe crossing 1|}} and {{tsl|en|Elbe crossing 2|}}). Assembly of lattice steel towers can be done using a [[起重机|crane]]. Lattice steel towers are generally made of angle-profiled [[steel beam]]s ([[L-beam|L-]] or {{tsl|en|T-beam|}}s). For very tall towers, [[桁架 (工程)]]es are often used.
[[als:Lichtgeschwindigkeit]]

[[am:የብርሃን ፍጥነት]]
==== Wood ====
[[ar:سرعة الضوء]]

[[an:Velocidat d'a luz]]
[[File:Electric power transmission - Ljusdal.JPG|thumb|Wood and metal crossbar]]
[[as:পোহৰৰ বেগ]]
[[File:InleUtilityPole.jpg|thumb|Wooden lattice transmission tower in [[茵萊湖]] ([[缅甸]]).]]
[[ast:Velocidá de la lluz]]
[[File:Transmission tower Mongolia.jpg|thumb|Simple wooden transmission tower in [[蒙古国]]]]
[[az:İşıq sürəti]]

[[bn:আলোর দ্রুতি]]
[[木材]] is a material which is limited in use in high-voltage transmission. Because of the limited height of available trees, the maximum height of wooden pylons is limited to approximately {{convert|30|m|ft|0|abbr=on}}. Wood is rarely used for lattice framework. Instead, they are used to build multi-pole structures, such as H-frame and K-frame structures. The voltages they carry are also limited, such as in other regions, where wood structures only carry voltages up to approximately 30 kV.
[[zh-min-nan:Kng-sok]]

[[be:Хуткасць святла]]
In countries such as Canada or the United States, wooden towers carry voltages up to 345 kV; these can be less costly than steel structures and take advantage of the surge voltage insulating properties of wood.<ref name=BEATY78>Donald Fink and Wayne Beaty (ed.) ''Standard Handbook for Electrical Engineers 11th Ed.'', Mc Graw Hill, 1978, {{ISBN|0-07-020974-X}}, pp. 14-102 and 14-103</ref> {{Asof|2012}}, 345 kV lines on wood towers are still in use in the US and some are still being constructed on this technology.<ref>http://www.spta.org/pdf/Reisdorff%20Lam%20%209-11.pdf</ref><ref>{{cite web|url=http://www.mainepower.com/winterport.htm|title=Winterport, Maine|author=Olive Development|publisher=}}</ref> Wood can also be used for temporary structures while constructing a permanent replacement.
[[be-x-old:Хуткасьць сьвятла]]

[[bg:Скорост на светлината]]
==== Concrete ====
[[bs:Brzina svjetlosti]]

[[br:Tizh ar gouloù]]
[[File:Beton-Dreiebenenmast.jpg|thumb|A reinforced concrete pole in Germany]]
[[ca:Velocitat de la llum]]

[[cs:Rychlost světla]]
[[混凝土]] pylons are used in [[德国国名]] normally only for lines with operating [[電壓]]s below 30 kV. In exceptional cases, concrete pylons are used also for 110 kV lines, as well as for the public grid or for the [[鐵路運輸]] traction current grid. In Switzerland, concrete pylons with heights of up to 59.5 metres (world's tallest pylon of prefabricated concrete at [[利陶]]) are used for 380 kV overhead lines. Concrete poles are also used in Canada and the United States.
[[cy:Cyflymder golau]]

[[da:Lysets hastighed]]
Concrete pylons, which are not prefabricated, are also used for constructions taller than 60 metres. One example is a {{convert|66|m|ft|0|abbr=on}} tall pylon of a 380 kV powerline near Reuter West Power Plant in Berlin. Such pylons look like industrial chimneys.{{citation needed|date=August 2017}} In China some pylons for lines crossing rivers were built of concrete. The tallest of these pylons belong to the Yangtze Powerline crossing at Nanjing with a height of {{convert|257|m|ft|0|abbr=on}}.
[[de:Lichtgeschwindigkeit]]

[[et:Valguse kiirus]]
=== Special designs ===
[[el:Ταχύτητα του φωτός]]

[[es:Velocidad de la luz]]

[[eo:Lumrapido]]
== Assembly ==
[[ext:Velociá de la lus]]

[[eu:Argiaren abiadura]]
[[File:Pylon cable riggers dismantling reel.JPG|thumb|Cable riggers atop a pylon engaged in adding a fiber optic data cable wound around the top tower stay cable. The cable {{tsl|en|Optical attached cable||(SkyWrap)}} is wound on by a traveling machine, which rotates a cable drum around the support cable as it goes. This travels under its own power from tower to tower, where it is dismantled and hoisted across to the opposite side. In the picture, the motor unit has been moved across but the cable drum is still on the arrival side.]]
[[fa:سرعت نور]]

[[fr:Vitesse de la lumière]]
Before transmission towers are even erected, prototype towers are tested at {{tsl|en|tower testing station|}}s. There are a variety of ways they can then be assembled and erected:
[[fur:Velocitât de lûs]]
[[Image:Hochspannungsbehelfsmast.jpg|thumb|upright|Temporary guyed pylon next to a commenced new tower]]
[[ga:Luas an tsolais]]
* They can be assembled horizontally on the ground and erected by push-pull cable. This method is rarely used because of the large assembly area needed.
[[gl:Velocidade da luz]]
* They can be assembled vertically (in their final upright position). Very tall towers, such as the {{tsl|en|Yangtze River Crossing|}}, were assembled in this way.
[[ko:빛의 속력]]
* A {{tsl|en|jin-pole|}} crane can be used to assemble lattice towers.<ref name=BTTi>{{cite web|author=Broadcast Tower Technologies|title=Gin Pole Services|url=http://www.tower-technologies.com/GinPole.htm|accessdate=2009-10-24}}</ref> This is also used for [[电线杆]]s.
[[hi:प्रकाश का वेग]]
* [[直升機]]s can serve as {{tsl|en|aerial crane|}}s for their assembly in areas with limited accessibility. Towers can also be assembled elsewhere and flown to their place on the transmission right-of-way.<ref>{{cite web|url=http://www.verticalmag.com/news/article/PoweringUp |archiveurl=https://web.archive.org/web/20151004113042/http://www.verticalmag.com/news/article/PoweringUp |title=Powering Up – Vertical Magazine|archivedate=4 October 2015|work=verticalmag.com |accessdate=4 October 2015 |url-status=live}}</ref> Helicopters may also be used for transporting disassembled towers for scrapping.<ref>{{cite web |title=Helicopter Transport of Transmission Towers |url=https://www.tdworld.com/td-how/helicopter-transport-transmission-towers |website=Transmission & Distribution World |date=21 May 2018}}</ref>
[[hr:Brzina svjetlosti]]

[[id:Laju cahaya]]

[[ia:Rapiditate de lumine]]
== Tower functions ==
[[is:Ljóshraði]]

[[it:Velocità della luce]]
[[File:Channel Island NT.jpg|thumb|Three-phase alternating current transmission towers over water, near [[达尔文 (澳大利亚)]], Australia]]
[[he:מהירות האור]]

[[ka:სინათლის სიჩქარე]]
Tower structures can be classified by the way in which they support the line conductors.<ref>American Society of Civil Engineers ''Design of latticed steel transmission structures'' ASCE Standard 10-97, 2000, {{ISBN|0-7844-0324-4}}, section C2.3</ref> Suspension structures support the conductor vertically using suspension insulators. Strain structures resist net tension in the conductors and the conductors attach to the structure through strain insulators. Dead-end structures support the full weight of the conductor and also all the tension in it, and also use strain insulators.
[[kk:Жарық жылдамдығы]]

[[sw:Kasi ya nuru]]
Structures are classified as tangent suspension, angle suspension, tangent strain, angle strain, tangent dead-end and angle dead-end.<ref name=BEATY78 /> Where the conductors are in a straight line, a tangent tower is used. Angle towers are used where a line must change direction.
[[la:Celeritas lucis]]

[[lv:Gaismas ātrums]]
=== Cross arms and conductor arrangement ===
[[lb:Liichtgeschwënnegkeet]]

[[lt:Šviesos greitis]]
Generally three conductors are required per AC 3-phase circuit, although single-phase and DC circuits are also carried on towers. Conductors may be arranged in one plane, or by use of several cross-arms may be arranged in a roughly symmetrical, triangulated pattern to balance the impedances of all three phases. If more than one circuit is required to be carried and the width of the line right-of-way does not permit multiple towers to be used, two or three circuits can be carried on the same tower using several levels of cross-arms. Often multiple circuits are the same voltage, but mixed voltages can be found on some structures.
[[jbo:nilsutra lo gusni]]

[[hu:Fénysebesség]]
== Other features ==
[[mk:Брзина на светлината]]

[[ml:പ്രകാശവേഗം]]

[[arz:سرعة النور]]

[[ms:Kelajuan cahaya]]
== 參見 ==
[[mn:Гэрлийн хурд]]
* [[电线杆]]
[[nl:Lichtsnelheid]]
* {{tsl|en|Live-line working|帶電工作}}
[[ja:光速]]

[[frr:Faard faan't laacht]]
== 參考資料 ==
[[no:Lysets hastighet]]
{{reflist|30em}}
[[nn:Ljosfarten]]

[[oc:Velocitat de la lutz]]
== 外部連結 ==
[[uz:Yorugʻlik tezligi]]

[[pnb:چانن دی دوڑ]]
{{Commons category|Electricity pylons}}
[[pl:Prędkość światła]]

[[pt:Velocidade da luz]]
* [http://www.pylons.org/ Pylon Appreciation Society]
[[ro:Viteza luminii]]
* [http://www.gorge.org/pylons Flash Bristow's pylon photo gallery and pylon FAQ]
[[rue:Швыдкость світла]]
* [http://www.magnificentviews.tk/ Magnificent Views: Pictures of High Voltage Towers (also offers technical information)]
[[ru:Скорость света]]
* [http://en.structurae.de/structures/ftype/index.cfm?id=2018 Structurae database of select notable transmission towers]
[[sco:Speed o licht]]
* [http://novoklimov.io.ua/ Pylons in Russia and other areas of former Soviet Union]
[[stq:Luchtgauegaid]]
* [https://www.bbc.co.uk/news/uk-32234656 Meet the 'pylon spotters' – BBC News]
[[sq:Shpejtësia e dritës]]

[[simple:Speed of light]]
{{Electricity generation}}
[[sk:Rýchlosť svetla]]

[[sl:Hitrost svetlobe]]
[[Category:輸電塔]]
[[ckb:خێرایی ڕووناکی]]
[[Category:输电]]
[[sr:Брзина светлости]]
[[Category:架空電纜]]
[[sh:Brzina svjetlosti]]
[[su:Laju cahaya]]
[[fi:Valonnopeus]]
[[sv:Ljusets hastighet]]
[[ta:ஒளியின் வேகம்]]
[[tt:Яктылык тизлеге]]
[[th:อัตราเร็วของแสง]]
[[tr:Işık hızı]]
[[uk:Швидкість світла]]
[[vi:Tốc độ ánh sáng]]
[[zh-classical:光速]]
[[war:Kalaksi han lamrag]]
[[zh-yue:光速]]
[[zh:光速]]

2020年8月17日 (一) 12:36的最新版本

電塔，又名輸電塔或輸電鐵塔，是用來承托架空電纜的結構物，通常為鋼製鐵塔-（英语：lattice tower）。輸電網路中的輸電系統主要用於大規模從發電廠輸送電力至負載中心，使用架空電纜相對地底電纜成本較低，故需要輸電塔將電纜抬高以避免高壓電力影響地面活動。較低電壓的配電系統的則常用电线杆作支撐物。電塔有各種不同形狀和大小，高度通常為15至55米之間，但最高可見於舟山島架空電纜（英语：Zhoushan Island Overhead Powerline Tie），當中有兩座370米高的輸電塔。除鋼鐵以外，亦有見以混凝土或木材作為建築材料。

電塔可主要分為三大類：懸吊塔（英语：suspension tower）、張力塔（英语：Dead-end tower）以及轉置塔（英语：transposition tower）。有些電塔則同時有以上數項塔種的功能。電塔和架空電纜為一種視覺污染（英语：visual pollution），故亦為管線地下化（英语：undergrounding）的其中一種理由。

結構

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電塔結構的建設費用通常佔該條輸電線路的三成至四成。其設計會因應地貌、氣候，以及架空電纜的電壓、線路數等參數而有所不同。跨臂

種類

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力學計算

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垂直負載

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縱向負載

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橫向負載

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線段跨度

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鋼構連接

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特殊設計

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Sometimes (in particular on steel lattice towers for the highest voltage levels) transmitting plants are installed, and antennas mounted on the top above or below the overhead ground wire. Usually these installations are for mobile phone services or the operating radio of the power supply firm, but occasionally also for other radio services, like directional radio. Thus transmitting antennas for low-power FM radio and television transmitters were already installed on pylons. On the Elbe Crossing 1 tower, there is a radar facility belonging to the 汉堡 water and navigation office.

For crossing broad valleys, a large distance between the conductors must be maintained to avoid short-circuits caused by conductor cables colliding during storms. To achieve this, sometimes a separate mast or tower is used for each conductor. For crossing wide rivers and straits with flat coastlines, very tall towers must be built due to the necessity of a large height clearance for navigation. Such towers and the conductors they carry must be equipped with flight safety lamps and reflectors.

Two well-known wide river crossings are the Elbe Crossing 1 and Elbe Crossing 2. The latter has the tallest overhead line masts in Europe, at 227米（745英尺） tall. In Spain, the overhead line crossing（英语：overhead line crossing） pylons in the Spanish bay of Cádiz（英语：Pylons of Cadiz） have a particularly interesting construction. The main crossing towers are 158米（518英尺） tall with one crossarm atop a 锥台 framework construction. The longest overhead line spans are the crossing of the Norwegian Sognefjord (4,597米（15,082英尺） between two masts) and the Ameralik Span（英语：Ameralik Span） in Greenland (5,376米（17,638英尺）). In Germany, the overhead line of the EnBW AG crossing of the Eyachtal has the longest span in the country at 1,444米（4,738英尺）.

In order to drop overhead lines into steep, deep valleys, inclined towers are occasionally used. These are utilized at the 胡佛水壩, located in the United States, to descend the cliff walls of the Black Canyon of the Colorado（英语：Black Canyon of the Colorado）. In Switzerland, a pylon inclined around 20 degrees to the vertical is located near 薩甘斯, St. Gallens. Highly sloping masts are used on two 380 kV pylons in Switzerland, the top 32 meters of one of them being bent by 18 degrees to the vertical.

Power station chimneys are sometimes equipped with crossbars for fixing conductors of the outgoing lines. Because of possible problems with corrosion by flue gases, such constructions are very rare.

A new type of pylon, called Wintrack pylons, will be used in the Netherlands starting in 2010. The pylons were designed as a minimalist structure by Dutch architects Zwarts and Jansma. The use of physical laws for the design made a reduction of the magnetic field possible. Also, the visual impact on the surrounding landscape is reduced.[1]

Two clown-shaped pylons appear in Hungary, on both sides of the M5 motorway, near 乌伊豪尔詹.[2]

The Pro Football Hall of Fame（英语：Pro Football Hall of Fame） in Canton, Ohio, U.S., and 美國電力公司 paired to conceive, design, and install goal post-shaped towers located on both sides of Interstate 77（英语：Interstate 77 in Ohio） near the hall as part of a power infrastructure upgrade.[3]

The Mickey Pylon（英语：Mickey Pylon） is a 米老鼠 shaped transmission tower on the side of 4號州際公路, near 華特迪士尼世界度假區 in 奥兰多 (佛罗里达州).

興建

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測試

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改建

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維修

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防墜裝置

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其他設置

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顏色

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Markers

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A typical tower identification tag

The 国际民用航空组织 issues recommendations on markers for towers and the conductors suspended between them. Certain jurisdictions will make these recommendations mandatory, for example that certain power lines must have overhead wire marker（英语：overhead wire marker）s placed at intervals, and that warning lights（英语：Aircraft warning lights） be placed on any sufficiently high towers,[4] this is particularly true of transmission towers which are in close vicinity to 機場s.

Electricity pylons often have an identification tag marked with the name of the line (either the terminal points of the line or the internal designation of the power company) and the tower number. This makes identifying the location of a fault to the power company that owns the tower easier.

Transmission towers, much like other steel lattice towers including broadcasting or cellphone towers, are marked with signs which discourage public access due to the danger of the high voltage. Often this is accomplished with a sign warning of the high voltage. At other times, the entire access point to the transmission corridor is marked with a sign.

絕緣子

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Arcing horns. Designs may vary.

架空電纜需與大地及電塔隔離以免短路，然而由於電塔需承托電纜無法使用空氣作為絕緣體，故需於承托處額外加上絕緣，通常為玻璃或陶瓷碟，稱之為絕緣子或礙子[5]。絕緣子的材質除上述的玻璃或陶瓷以外，亦有矽氧樹脂或EPDM橡膠（英语：EPDM rubber）等複合材料。絕緣子以串聯型式將架空電纜連接至電塔，而其數量會因電壓和環境因素而增加，例如11千伏線路會有一至兩隻絕緣子，400千伏線路則可達20隻絕緣子[6]。絕緣子的形狀增加了絕緣體表面的長度，由此減少了潮濕時短路或漏電的機會。

架空線減震器

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Stockbridge damper bolted to line close to the point of attachment to the tower. It prevents mechanical vibration building up in the line.

架空線減震器（英语：Stockbridge damper）s are added to the transmission lines a meter or two from the tower. They consist of a short length of cable clamped in place parallel to the line itself and weighted at each end. The size and dimensions are carefully designed to damp any buildup of mechanical oscillation of the lines that could be induced by mechanical vibration most likely that caused by wind. Without them its possible for a standing wave to become established that grows in magnitude and destroys the line or the tower.

Arcing horns

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Arcing horns（英语：Arcing horns） are sometimes added to the ends of the insulators in areas where voltage surges may occur. These may be caused by either lightning strikes or in switching operations. They protect power line insulators from damage due to arcing. They can be seen as rounded metal pipework at either end of the insulator and provide a path to earth in extreme circumstances without damaging the insulator.

Physical security

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Towers will have a level of physical security to prevent members of the public or climbing animals from ascending them. This may take the form of a security fence or climbing baffles added to the supporting legs. Some countries require that lattice steel towers be equipped with a 有刺铁丝网 barrier approximately 3米（9.8英尺） above ground in order to deter unauthorized climbing. Such barriers can often be found on towers close to roads or other areas with easy public access, even where there is not a legal requirement. In the United Kingdom, all such towers are fitted with barbed wire.







High voltage AC transmission towers

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Single-circuit three-phase transmission line

三相電 systems are used for high voltage (66- or 69-kV and above) and extra-high voltage (110- or 115-kV and above; most often 138- or 230-kV and above in contemporary systems) AC transmission lines. In some European countries, e.g. Germany, Spain or Czech Republic, smaller lattice towers are used for medium voltage (above 10 kV) transmission lines too. The towers must be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or 桁架 (工程)es (wooden structures are used in Canada, Germany, and 斯堪的纳维亚 in some cases) and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or EPDM rubber（英语：EPDM rubber） material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.

Typically, one or two ground wires, also called "guard" wires, are placed on top to intercept lightning and harmlessly divert it to ground.

Towers for high- and extra-high voltage are usually designed to carry two or more electric circuits (with very rare exceptions, only one circuit for 500-kV and higher).[來源請求] If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction. Indeed, for economic reasons, some transmission lines are designed for three (or four) circuits, but only two (or three) circuits are initially installed.

Some high voltage circuits are often erected on the same tower as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for 電氣化鐵路.

High voltage DC transmission towers

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HVDC distance tower near the terminus of the Nelson River Bipole（英语：Nelson River Bipole） adjacent to Dorsey Converter Station near Rosser, Manitoba（英语：Rosser, Manitoba）, Canada — August 2005

高壓直流輸電 (HVDC) transmission lines are either monopolar or bipolar systems. With bipolar systems, a conductor arrangement with one conductor on each side of the tower is used. On some schemes, the ground conductor is used as electrode line（英语：electrode line） or ground return. In this case, it had to be installed with insulators equipped with surge arrestors on the pylons in order to prevent electrochemical corrosion of the pylons. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, often conductors on both sides of the tower are installed for mechanical reasons. Until the second pole is needed, it is either used as electrode line or joined in parallel with the pole in use. In the latter case, the line from the converter station to the earthing (grounding) electrode is built as underground cable, as overhead line on a separate right of way or by using the ground conductors.

Electrode line towers are used in some HVDC schemes to carry the power line from the converter station to the grounding electrode. They are similar to structures used for lines with voltages of 10–30 kV, but normally carry only one or two conductors.

AC transmission towers may be converted to full or mixed HVDC use, to increase power transmission levels at a lower cost than building a new transmission line.[7][8]

Railway traction line towers

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Tension tower with phase transposition of a powerline for single-phase AC（英语：Single-phase generator） traction current (110 kV, 16.67 Hz) near 巴托洛梅, Germany

Towers used for single-phase AC（英语：Single-phase generator） 鐵路運輸 traction lines are similar in construction to those towers used for 110 kV three-phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). These are usually arranged on one level, whereby each circuit occupies one half of the cross arm. For four traction circuits, the arrangement of the conductors is in two levels and for six electric circuits, the arrangement of the conductors is in three levels.

Towers for different types of currents

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Pylon in Sweden about 1918.

AC circuits of different frequency and phase-count, or AC and DC circuits, may be installed on the same tower. Usually all circuits of such lines have voltages of 50 kV and more. However, there are some lines of this type for lower voltages. For example, towers used by both railway traction power circuits and the general three-phase AC grid.

Two very short sections of line carry both AC and DC power circuits. One set of such towers is near the terminal of HVDC Volgograd-Donbass（英语：HVDC Volgograd-Donbass） on Volga Hydroelectric Power Station. The other are two towers south of Stenkullen, which carry one circuit of HVDC Konti-Skan and üne circuit of the three-phase AC line Stenkullen-Holmbakullen.

Towers carrying AC circuits and DC electrode lines exist in a section of the powerline between Adalph Static Inverter Plant and Brookston the pylons carry the electrode line of HVDC Square Butte（英语：Square Butte (transmission line)）.

The electrode line of HVDC CU（英语：CU (Powerline)） at the converter station at Coal Creek Station uses on a short section the towers of two AC lines as support.

The overhead section of the electrode line（英语：electrode line） of Pacific DC Intertie（英语：Pacific DC Intertie） from Sylmar Converter Station to the grounding electrode in the Pacific Ocean near Will Rogers State Beach（英语：Will Rogers State Beach） is also installed on AC pylons. It runs from Sylmar East Converter Station to Southern California Edison Malibu Substation, where the overhead line section ends.

In Germany, Austria and Switzerland some transmission towers carry both public AC grid circuits and railway traction power in order to better use rights of way.

Tower designs

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Shape

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Guyed "Delta" transmission tower (a combination of guyed "V" and "Y") in 内华达州.

Different shapes of transmission towers are typical for different countries. The shape also depends on voltage and number of circuits.

One circuit

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Delta pylons are the most common design for single circuit lines, because of their stability. They have a V-shaped body with a horizontal arm on the top, which forms an inverted Delta. Larger Delta towers usually use two guard cables.

Portal pylons are widely used in Ireland, Scandinavia and Canada. They stand on two legs with one cross arm, which gives them a H-shape. Up to 110 kV they often were made from wood, but higher voltage lines use steel pylons.

Smaller single circuit pylons may have two small cross arms on one side and one on the other.

Two circuits

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One level pylons only have one cross arm carrying 3 cables on each side. Sometimes they have an additional cross arm for the protection cables. They are frequently used close to airports due to their reduced height.

Typical T-shaped 110 kV tower from the former 東德.

Danube pylons or Donaumasten got their name from a line built in 1927 next to the 多瑙河. They are the most common design in central European countries like Germany or Poland. They have two cross arms, the upper arm carries one and the lower arm carries two cables on each side. Sometimes they have an additional cross arm for the protection cables.

Ton shaped towers are the most common design, they have 3 horizontal levels with one cable very close to the pylon on each side. In the United Kingdom the second level is often (but not always) wider than the other ones while in the United States all cross arms have the same width.

A close up of the wires attached to the pylon, showing the various parts annotated.

Four circuits

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Christmas-tree-shaped towers for 4 or even 6 circuits are common in Germany and have 3 cross arms where the highest arm has each one cable, the second has two cables and the third has three cables on each side. The cables on the third arm usually carry circuits for lower high voltage.

Support structures

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Danube pole for 110 kV in Germany, built in the 1930s

Towers may be self-supporting and capable of resisting all forces due to conductor loads, unbalanced conductors, wind and ice in any direction. Such towers often have approximately square bases and usually four points of contact with the ground.

A semi-flexible tower is designed so that it can use overhead grounding wires to transfer mechanical load to adjacent structures, if a phase conductor breaks and the structure is subject to unbalanced loads. This type is useful at extra-high voltages, where phase conductors are bundled (two or more wires per phase). It is unlikely for all of them to break at once, barring a catastrophic crash or storm.

A guyed mast（英语：guyed mast） has a very small footprint and relies on guy wires in tension to support the structure and any unbalanced tension load from the conductors. A guyed tower can be made in a V shape, which saves weight and cost.[9]

Materials

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Tubular steel

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Steel tube tower next to older lattice tower near 沃加沃加, Australia

Poles made of tubular 钢 generally are assembled at the factory and placed on the right-of-way afterward. Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer the use of monopolar steel or concrete towers over lattice steel for new power lines and tower replacements. [來源請求]

In Germany（英语：Energy in Germany） steel tube pylons are also established predominantly for medium voltage lines, in addition, for high voltage transmission lines or two electric circuits for operating voltages by up to 110 kV. Steel tube pylons are also frequently used for 380 kV lines in France（英语：Energy in France）, and for 500 kV lines in the United States（英语：Energy in the United States）.

Lattice

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参见：Lattice tower

A lattice tower is a framework construction made of steel or aluminium sections. Lattice towers are used for power lines of all voltages, and are the most common type for high-voltage transmission lines. Lattice towers are usually made of galvanized steel. Aluminium is used for reduced weight, such as in mountainous areas where structures are placed by helicopter. Aluminium is also used in environments that would be corrosive to steel. The extra material cost of aluminium towers will be offset by lower installation cost. Design of aluminium lattice towers is similar to that for steel, but must take into account aluminium's lower 杨氏模量.

A lattice tower is usually assembled at the location where it is to be erected. This makes very tall towers possible, up to 100米（328英尺） (and in special cases even higher, as in the Elbe crossing 1（英语：Elbe crossing 1） and Elbe crossing 2（英语：Elbe crossing 2）). Assembly of lattice steel towers can be done using a crane. Lattice steel towers are generally made of angle-profiled steel beams (L- or T-beam（英语：T-beam）s). For very tall towers, 桁架 (工程)es are often used.

Wood

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Wood and metal crossbar
Wooden lattice transmission tower in 茵萊湖 (缅甸).
Simple wooden transmission tower in 蒙古国

木材 is a material which is limited in use in high-voltage transmission. Because of the limited height of available trees, the maximum height of wooden pylons is limited to approximately 30米（98英尺）. Wood is rarely used for lattice framework. Instead, they are used to build multi-pole structures, such as H-frame and K-frame structures. The voltages they carry are also limited, such as in other regions, where wood structures only carry voltages up to approximately 30 kV.

In countries such as Canada or the United States, wooden towers carry voltages up to 345 kV; these can be less costly than steel structures and take advantage of the surge voltage insulating properties of wood.[9] 截至2012年 (2012-Missing required parameter 1=month!), 345 kV lines on wood towers are still in use in the US and some are still being constructed on this technology.[10][11] Wood can also be used for temporary structures while constructing a permanent replacement.

Concrete

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A reinforced concrete pole in Germany

混凝土 pylons are used in 德国国名 normally only for lines with operating 電壓s below 30 kV. In exceptional cases, concrete pylons are used also for 110 kV lines, as well as for the public grid or for the 鐵路運輸 traction current grid. In Switzerland, concrete pylons with heights of up to 59.5 metres (world's tallest pylon of prefabricated concrete at 利陶) are used for 380 kV overhead lines. Concrete poles are also used in Canada and the United States.

Concrete pylons, which are not prefabricated, are also used for constructions taller than 60 metres. One example is a 66米（217英尺） tall pylon of a 380 kV powerline near Reuter West Power Plant in Berlin. Such pylons look like industrial chimneys.[來源請求] In China some pylons for lines crossing rivers were built of concrete. The tallest of these pylons belong to the Yangtze Powerline crossing at Nanjing with a height of 257米（843英尺）.

Special designs

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Assembly

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Cable riggers atop a pylon engaged in adding a fiber optic data cable wound around the top tower stay cable. The cable (SkyWrap)（英语：Optical attached cable） is wound on by a traveling machine, which rotates a cable drum around the support cable as it goes. This travels under its own power from tower to tower, where it is dismantled and hoisted across to the opposite side. In the picture, the motor unit has been moved across but the cable drum is still on the arrival side.

Before transmission towers are even erected, prototype towers are tested at tower testing station（英语：tower testing station）s. There are a variety of ways they can then be assembled and erected:

Temporary guyed pylon next to a commenced new tower
  • They can be assembled horizontally on the ground and erected by push-pull cable. This method is rarely used because of the large assembly area needed.
  • They can be assembled vertically (in their final upright position). Very tall towers, such as the Yangtze River Crossing（英语：Yangtze River Crossing）, were assembled in this way.
  • A jin-pole（英语：jin-pole） crane can be used to assemble lattice towers.[12] This is also used for 电线杆s.
  • 直升機s can serve as aerial crane（英语：aerial crane）s for their assembly in areas with limited accessibility. Towers can also be assembled elsewhere and flown to their place on the transmission right-of-way.[13] Helicopters may also be used for transporting disassembled towers for scrapping.[14]


Tower functions

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Three-phase alternating current transmission towers over water, near 达尔文 (澳大利亚), Australia

Tower structures can be classified by the way in which they support the line conductors.[15] Suspension structures support the conductor vertically using suspension insulators. Strain structures resist net tension in the conductors and the conductors attach to the structure through strain insulators. Dead-end structures support the full weight of the conductor and also all the tension in it, and also use strain insulators.

Structures are classified as tangent suspension, angle suspension, tangent strain, angle strain, tangent dead-end and angle dead-end.[9] Where the conductors are in a straight line, a tangent tower is used. Angle towers are used where a line must change direction.

Cross arms and conductor arrangement

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Generally three conductors are required per AC 3-phase circuit, although single-phase and DC circuits are also carried on towers. Conductors may be arranged in one plane, or by use of several cross-arms may be arranged in a roughly symmetrical, triangulated pattern to balance the impedances of all three phases. If more than one circuit is required to be carried and the width of the line right-of-way does not permit multiple towers to be used, two or three circuits can be carried on the same tower using several levels of cross-arms. Often multiple circuits are the same voltage, but mixed voltages can be found on some structures.

Other features

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參見

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參考資料

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  1. ^ New High Voltage Pylons for the Netherlands. 2009 [2010-04-24]. 
  2. ^ Clown-shaped High Voltage Pylons in Hungary. 47°14′09″N 19°23′27″E / 47.2358442°N 19.3907302°E / 47.2358442; 19.3907302 (Clown-shaped pylon)
  3. ^ Rudell, Tim. Drive Through Goal Posts at the Pro Football Hall of Fame. WKSU（英语：WKSU）. 2016-06-28 [2019-07-14]. 40°49′03″N 81°23′48″W / 40.8174274°N 81.3966678°W / 40.8174274; -81.3966678 (Goal post pylons)
  4. ^ Chapter 6. Visual aids for denoting obstacles (PDF). Annex 14 Volume I Aerodrome design and operations. 国际民用航空组织: 6–3, 6–4, 6–5. 2004-11-25 [1 June 2011]. 6.2.8 ... spherical ... diameter of not less than 60 cm. ... 6.2.10 ... should be of one colour. ... Figure 6-2 ... 6.3.13 
  5. ^ CLP 中電. 唔准諗即刻答！知唔知圖中嗰串碟仔係乜？. Facebook. CLP 中電. 2017-09-27 [2020-08-16]. 
  6. ^ CLP 中電. 唔准諗即刻答！知唔知圖中嗰串碟仔係乜？. Facebook. CLP 中電. 2018-11-09 [2020-08-16]. 
  7. ^ Convert from AC to HVDC for higher power transmission. ABB Review. 2018: 64–69 [20 June 2020]. 
  8. ^ Liza Reed; Granger Morgan; Parth Vaishnav; Daniel Erian Armanios. Converting existing transmission corridors to HVDC is an overlooked option for increasing transmission capacity. Proceedings of the National Academy of Sciences. 9 July 2019, 116 (28): 13879–13884. PMC 6628792 可免费查阅. PMID 31221754. doi:10.1073/pnas.1905656116. 
  9. ^ 9.0 9.1 9.2 Donald Fink and Wayne Beaty (ed.) Standard Handbook for Electrical Engineers 11th Ed., Mc Graw Hill, 1978, ISBN 0-07-020974-X, pp. 14-102 and 14-103
  10. ^ http://www.spta.org/pdf/Reisdorff%20Lam%20%209-11.pdf
  11. ^ Olive Development. Winterport, Maine. 
  12. ^ Broadcast Tower Technologies. Gin Pole Services. [2009-10-24]. 
  13. ^ Powering Up – Vertical Magazine. verticalmag.com. [4 October 2015]. （原始内容存档于4 October 2015）. 
  14. ^ Helicopter Transport of Transmission Towers. Transmission & Distribution World. 21 May 2018. 
  15. ^ American Society of Civil Engineers Design of latticed steel transmission structures ASCE Standard 10-97, 2000, ISBN 0-7844-0324-4, section C2.3

外部連結

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维基共享资源上的相关多媒体资源：JC1/twenty-second
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