Examine individual changes

'{{About|a specific type of [[transformer]]|the more general electrical component|Inductor}} [[File:Induktionsapparat hg.jpg|thumb|upright=1.2|Antique induction coil used in schools, from around 1900, Bremerhaven, Germany]] [[File:Induction coil cutaway.jpg|thumb|upright=1.2|Induction coil showing construction, from 1920.]] An '''induction coil''' or "spark coil" ([[archaism|archaically]] known as an '''inductorium''' or '''Ruhmkorff coil'''<ref name="Britannica" /> after [[Heinrich Ruhmkorff]]) is a type of electrical [[transformer]] used to produce high-voltage pulses from a low-voltage [[direct current]] (DC) supply.<ref name="Britannica">John Archibald Fleming {{cite encyclopedia | title = Induction Coil | encyclopedia = Encyclopaedia Britannica, 11th Ed. | volume = 13 | pages = 502–505 | publisher = The Encyclopaedia Britannica Co. | date = 1911 | url = http://books.google.com/books?id=XFobAQAAMAAJ&pg=RA1-PA504&dq=%22induction+coil | id = | accessdate = October 13, 2014}}</ref><ref name="Collins">{{cite book |last=Collins |first=Archie F. |title=The Design and Construction of Induction Coils |date=1908 |page= |publisher=Munn & Co.|location=New York|url=http://books.google.com/?id=dJNPAAAAMAAJ&pg=PA98}} p.98</ref> To create the flux changes necessary to induce voltage in the secondary, the direct current in the primary is repeatedly interrupted by a vibrating mechanical [[Switch|contact]] called an [[interrupter]].<ref name="Britannica" /> Developed beginning in 1836 by [[Nicholas Callan]] and others,<ref name="Britannica" /> the induction coil was the first type of transformer. They were widely used in [[x-ray machine]]s,<ref name="Britannica" /><ref name="Collins1">[http://books.google.com/books?id=dJNPAAAAMAAJ&pg=PR3&dq=X-ray Collins, 1908, p. iii]</ref> [[spark-gap transmitter|spark-gap radio transmitter]]s,<ref name="Britannica" /><ref name="Collins1" /> [[arc lamp|arc lighting]] and quack medical [[electrotherapy]] devices from the 1880s to the 1920s. Today their only common use is as the [[ignition coil]]s in [[internal combustion engine]]s, and in physics education to demonstrate [[Faraday's law of induction|induction]]. The term 'induction coil' is also used for a coil carrying high-frequency [[alternating current]] (AC), producing eddy currents to heat objects placed in the interior of the coil, in [[induction heating]] or [[zone melting]] equipment. ==Construction and function== [[Image:Ruhmkorff coil schematic 1.svg|thumb|upright=1.3|Schematic diagram]] See schematic diagram. An induction coil consists of two coils of insulated copper wire wound around a common [[magnetic core|iron core]] ''(M)''.<ref name="Britannica" /><ref name="Collins2">[http://books.google.com/books?id=dJNPAAAAMAAJ&pg=PA16 Collins, 1908, p. 16-19]</ref> One coil, called the ''[[primary winding]]'' ''(P)'', is made from relatively few (tens or hundreds) turns of coarse wire.<ref name="Collins2" /> The other coil, the ''[[secondary winding]],'' ''(S)'' typically consists of many (thousands) turns of fine wire.<ref name="Britannica" /><ref name="Collins2" /> An [[electric current]] is passed through the primary, creating a [[magnetic field]].<ref name="Britannica" /><ref name="Collins2" /> Because of the common core, most of the primary's magnetic field couples with the secondary winding. The primary behaves as an [[inductor]], storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a [[high voltage]] pulse to be developed across the secondary terminals through [[electromagnetic induction]]. Because of the large number of turns in the secondary coil, the secondary voltage pulse is typically many thousands of [[volt]]s. This voltage is often sufficient to cause an [[electric spark]], to jump across an air gap ''(G)'' separating the secondary's output terminals. For this reason, induction coils were called spark coils. The size of induction coils is usually specified by the length of spark it can produce; a '4&nbsp;inch' (10&nbsp;cm) induction coil is one that could produce a 4&nbsp;inch arc. ===The interrupter=== {{multiple image | align = left | direction = vertical | header = | image1 = Induction coil waveforms.svg | caption1 = Without capacitor | image2 = Induction coil waveforms with capacitor.svg | caption2 = With capacitor | width = 200 | footer = Waveforms in the induction coil. <span style="color:blue;">''i''₁ ''(blue)''</span> is the current in the coil's primary winding, <span style="color:red;">''v''₂ ''(red)''</span> is the secondary voltage. Not to common scale; ''v''₂ is much larger in bottom drawing. }} To operate the coil continuously, the DC supply current must be broken repeatedly to create the magnetic field changes needed for induction.<ref name="Britannica" /> Induction coils use a magnetically activated vibrating arm called an ''interrupter'' or ''break'' ''(A)'' to rapidly connect and break the current flowing into the primary coil.<ref name="Britannica" /> The interrupter is mounted on the end of the coil next to the iron core. When the power is turned on, the magnetic field of the core created by the current flowing in the primary attracts the interrupter's iron armature ''(A)'' attached to the springy arm, opening a pair of contacts ''(K)'' in the primary circuit. When the magnetic field then collapses, the arm springs away, closing the contacts again and turning on the current again. yhggjhghjhjbnbmnbhhvhvnbvvvvvvvvvvvvvvvvvvvvvvvnnhhhhhhhhhh The magnetic field pulls on the armature again, opening the contacts again. This cycle is repeated many times per second. The secondary voltage ''v''₂ ''(<span style="color:red;">red</span>, left)'', is roughly proportional to the rate of change of primary current ''i''₁ (<span style="color:blue;">blue</span>)''. Opposite potentials are induced in the secondary when the interrupter 'breaks' the circuit and 'closes' the circuit. However, the current change in the primary is much more abrupt when the interrupter 'breaks'. When the contacts close, the current builds up slowly in the primary because the supply voltage has a limited ability to force current through the coil's inductance. In contrast, when the interrupter contacts open, the current falls to zero suddenly. So the pulse of voltage induced in the secondary at 'break' is much larger than the pulse induced at 'close', it is the 'break' that generates the coil's high voltage output. ===Capacitor=== An arc forms at the interrupter contacts at 'break' which consumes energy stored in the coil, slowing the rate of change of primary current, reducing the output voltage. To prevent this a [[capacitor]] ''(C)'' of 0.5 to 15 [[Microfarad|μF]] is connected across the contacts to increase the speed of switching on 'break', producing much higher voltages. It also prevents damage to the contacts by the arc. The capacitor and primary winding together form a [[tuned circuit]], so on break an oscillating decaying sinusoidal current flows in the primary. This induces a sinusoidal voltage in the secondary. So the high voltage output pulse at each break actually consists of a rapidly alternating series of positive and negative pulses ''(left)'' which decay rapidly to zero, each pulse causing a separate spark between the output electrodes. ===Construction details=== To prevent the high voltages generated in the coil from breaking down the thin [[Electrical insulation|insulation]] and [[Electric arc|arcing]] between the secondary wires, the secondary coil uses special construction so as to avoid having wires carrying large voltage differences lying next to each other. In one widely used technique, the secondary coil is wound in many thin flat pancake-shaped sections (called "pies"), connected in [[Series circuit|series]].<ref name="Britannica" /> The primary coil is first wound on the iron core, and insulated from the secondary with a thick paper or rubber coating.<ref name="Britannica" /> Then each secondary subcoil is connected to the coil next to it, and slid onto the iron core, insulated from adjoining coils with waxed cardboard disks. The voltage developed in each subcoil isn't large enough to jump between the wires in the subcoil.<ref name="Britannica" /> Large voltages are only developed across many subcoils in series, which are too widely separated to arc over. To give the entire coil a final insulating coating, it is immersed in melted [[paraffin wax]] or [[rosin]], and the air evacuated to ensure there are no air bubbles left inside, and the paraffin allowed to solidify, so the entire coil is encased in wax. To prevent [[eddy current]]s, which cause energy losses, the iron core is made of a bundle of parallel iron wires, individually coated with [[shellac]] to insulate them electrically.<ref name="Britannica" /> The eddy currents, which flow in loops in the core perpendicular to the magnetic axis, are blocked by the layers of insulation. The ends of the insulated primary coil often protruded several inches from either end of the secondary coil, to prevent arcs from the secondary to the primary or the core. ==Mercury and electrolytic interrupters== {{multiple image | align = right | direction = horizontal | header = | image1 = Wehnelt interrupter.png | width1 = 130 | image2 = Mercury turbine interrupter.png | width2 = 200 | footer = ''(left)'' 3-electrode Wehnelt interrupter used in high power coils. ''(right)'' Mercury turbine interrupter. The motor turns the toothed wheel while a stream of mercury is sprayed on the teeth. By adjusting the wheel up and down the duty cycle of the primary current can be changed.}} Although modern induction coils used for educational purposes all use the vibrating arm 'hammer' type interrupter described above, these were inadequate for powering the large induction coils used in [[spark-gap transmitter|spark-gap radio transmitters]] and [[x-ray machine]]s around the turn of the 20th century. In powerful coils the high primary current created arcs at the interrupter contacts which quickly destroyed the contacts.<ref name="Britannica" /> Also, since each "break" produces a pulse of voltage from the coil, the more breaks per second the greater the power output. Hammer interrupters were not capable of interruption rates over 200 breaks per second, and the ones used on powerful coils were limited to 20 – 40 breaks per second. Therefore much research went into improving interrupters, and improved designs were used in high power coils, with the hammer interrupters only used on small coils under 8" sparks.<ref>[http://books.google.com/?id=dJNPAAAAMAAJ&pg=PA98 Collins, 1908, p. 98]</ref> [[Léon Foucault]] and others developed interrupters consisting of an oscillating needle dipping into and out of a container of [[mercury (element)|mercury]].<ref name="Britannica" /> The mercury was covered with a layer of spirits which extinguished the arc quickly, causing faster switching. These were often driven by a separate electromagnet or motor,<ref name="Britannica" /> which allowed the interruption rate and "dwell" time to be adjusted separately from the primary current. The largest coils used either electrolytic or mercury turbine interrupters.<ref name="Britannica" /> The electrolytic or Wehnelt interrupter, invented by [[Arthur Wehnelt]] in 1899, consisted of a short [[platinum]] needle [[anode]] immersed in an [[electrolyte]] of dilute [[sulfuric acid]], with the other side of the circuit connected to a lead plate [[cathode]].<ref name="Britannica" /><ref name="Moore"/> When the primary current passed through it, hydrogen gas bubbles formed on the needle which repeatedly broke the circuit. This resulted in a primary current broken randomly at rates up to 2000 breaks per second. They were preferred for powering X-ray tubes. They produced a lot of heat, and due to the hydrogen could explode. Mercury turbine interrupters had a [[Centrifugal force|centrifugal]] pump which sprayed a stream of liquid [[mercury (element)|mercury]] onto rotating metal contacts.<ref name="Britannica" /> They could achieve interruption rates up to 10,000 breaks per second, and were the most widely used type of interrupter in commercial wireless stations.<ref name="Britannica" /><ref name="Moore">{{cite book | last = Moore | first = Arthur | authorlink = | title = How to make a wireless set | publisher = The Popular Mechanics Co. | date = 1911 | location = Chicago | url = http://archive.org/details/howtomakewireles00more | doi = | id = |quote=The electrolytic interrupter consists of a vessel containing a solution of dilute sulphuric acid with two terminals immersed in this solution. The positive terminal or anode is made of platinum and should have a surface of about 3/16 in.[sic] The negative terminal or cathode is made of lead and should have an area of something like 1 sq. ft. When this interrupter is connected in series with the primary of an induction coil and a source of electromotive force of about 40 volts, the circuit will be interrupted, due to the formation and collapse of bubbles on the platinum electrode. | isbn = 1440048746}} Page 31 describes electrolytic interrupter, but does not identify as Wehnelt interrupter.</ref> {{multiple image | align = center |direction = horizontal | image1 = William Sturgeons first induction coil.png | caption1 = Early coil by [[William Sturgeon]], 1837. The sawtooth zinc interrupter wheel ''(D)'' was turned by hand. The first coil to use a divided core of iron wires ''(F)'' to prevent eddy currents. | width1 = 133 | image2 = NMAH DC - IMG 8815.JPG | caption2 = Early coil by Charles G. Page, 1838, had one of the first automatic interrupters. The cup was filled with mercury. The magnetic field attracted the iron piece on the arm ''(left)'', lifting the wire out of the cup, breaking the primary circuit. | width2 = 150 | image3 = Ruhmkorff coil.png | caption3 = Induction coil by [[Heinrich Ruhmkorff]], 1850s. In addition to the hammer interrupter ''(right)'', it had a mercury interrupter by Fizeau ''(left)'' that could be adjusted to change the dwell time. | width3 = 255 | image4 = Large Apps-Spottiswoode induction coil.png | caption4 = One of the largest coils ever constructed, built in 1877 by Alfred Apps for William Spottiswoode. Wound with 280 miles of wire, could produce a 42 in. (106 cm) spark, corresponding to roughly one million volts. Powered by 30 quart size liquid batteries and a separate interrupter ''(not shown)''. | width4 = 210 | image5 = Vibrator ignition coil.png | caption5 = [[Trembler coil|Vibrator ignition coil]] used in early automobiles such as the Ford Model T around 1910 | width5 = 135 | image6 = Igncoil.jpg | caption6 = Modern automobile [[ignition coil]], the largest remaining use for induction coils | width6 = 120 }} ==History== [[Image:Callan's first induction coil.jpg|thumb|upright=0.66|The first induction coil, built by Nicholas Callan, 1836.]] The induction coil was the first type of electrical [[transformer]]. During its development between 1836 and the 1860s, mostly by trial and error, researchers discovered many of the principles that governed all transformers, such as the proportionality between turns and output voltage, and the use of a "divided" iron core to reduce [[eddy current]] losses. [[Michael Faraday]] discovered the principle of induction, [[Faraday's induction law]], in 1831 and did the first experiments with induction between coils of wire.<ref>{{cite journal|last=Faraday|first=Michael|date=1834|title=Experimental researches on electricity, 7th series|journal=Phil. Trans. R. Soc. (London)|volume=124|pages=77–122|doi=10.1098/rstl.1834.0008}}</ref> The induction coil was invented by the Irish scientist and Catholic priest [[Nicholas Callan]] in 1836 at the [[National University of Ireland, Maynooth|St. Patrick's College, Maynooth]]<ref name="Britannica" /><ref name="Callan1">{{cite journal | last = Callan | first = N. J. | authorlink = | title = On a new galvanic battery | journal = Philosophical Magazine | volume = 9 | issue = 3 | pages = 472–478 | publisher = | location = | date = December 1836 | url = http://books.google.com/books?id=c5IOAAAAIAAJ&pg=PA477&lpg=PA477#v=onepage&q&f=false | issn = | doi = 10.1080/14786443608649044 | id = | accessdate = February 14, 2013}}</ref><ref name="Sturgeon">Callan, N. J. ''A Description of an Electromagnetic Repeater'' in {{cite book | last = Sturgeon, Ed. | first = William | title = The Annals of Electricity, Magnetism, and Chemistry, Vol. 1 | publisher = Sherwood, Gilbert, and Piper | date = 1837 | location = London | pages = 229–230 | url = http://books.google.com/books?id=SXgMAAAAYAAJ&pg=PA229&lpg=PA229 | doi = | id = | isbn = }} and p.522 fig. 52</ref><ref name="Fleming">{{cite book | last = Fleming | first = John Ambrose | title = The Alternate Current Transformer in Theory and Practice, Vol. 2 | publisher = The Electrician Publishing Co. | date = 1896 | location = London | pages = 16–18 | url = http://books.google.com/?id=17sKAAAAIAAJ&pg=PA16 | doi = | id = | isbn = }}</ref><ref name="NationalScienceMuseum">{{cite web | last = McKeith | first = Niall | title = Reverend Professor Nicholas Callan | work = National Science Museum | publisher = St. Patrick's College, Maynooth | url = http://www.nuim.ie/museum/ncallan.html | doi = | accessdate = February 14, 2013}}</ref> and improved by [[William Sturgeon]] and [[Charles Grafton Page]].<ref name="Britannica" /> [[George Henry Bachhoffner]]<ref name="Britannica" /> and Sturgeon (1837) independently discovered that a "divided" iron core of iron wires reduced power losses.<ref name="Fleming2">[http://books.google.com/?id=17sKAAAAIAAJ&pg=PA10 Fleming (1896) ''The Alternate Current Transformer in Theory and Practice, Vol. 2'', p. 10-11]</ref> The early coils had hand cranked interrupters, invented by Callan and Antoine Philibert Masson (1837).<ref name="Masson1">{{cite journal | last = Masson | first = Antoine Philibert | authorlink = | title = Rapport sur plusieurs mémoires, relatifs à un mode particulier d'action des courants électriques (Report on several memoirs regarding a particular mode of action of electric currents) | journal = Comptes rendus | volume = 4 | issue = | pages = 456–460 | publisher = Elsevier | location = Paris | date = 1837 | url = http://gallica.bnf.fr/ark:/12148/bpt6k29634/f460.image | issn = | doi = | id = | accessdate = February 14, 2013}} On page 458, an interrupter consisting of a toothed wheel is described.</ref><ref name="Masson2">{{cite journal | last = Masson | first = A. | title = De l'induction d'un courant sur lui-même (On the induction of a current in itself) | journal = Annales de Chimie et de Physique | volume = 66 | issue = | pages = 5–36 | publisher = Elsevier | location = Paris | date = 1837 | url = http://books.google.com/books?id=GsxGpge1LEoC&pg=PA11#v=onepage&q&f=false | issn = | doi = | id = | accessdate = February 14, 2013}}</ref><ref name="MassonBreguet">{{cite journal | last = Masson | first = Antoine Philibert |author2=Louis Breguet | title = Mémoire sur l'induction | journal = Annales de chimie et de physique | volume = 4 | issue = 3 | pages = 129–152 | publisher = Elsevier | location = Paris | date = 1841 | url = http://books.google.com/books?id=UBwzAQAAMAAJ&pg=RA1-PA129#v=onepage&q&f=false | issn = | doi = | id = | accessdate = February 14, 2013}} On page 134, Masson describes the toothed wheels that functioned as an interrupter.</ref> The automatic 'hammer' interrupter was invented by Rev. Prof. [[James William MacGauley]] (1838) of Dublin, Ireland,<ref name="McGauley">{{cite journal | last = McGauley | first = J. W. | title = Electro-magnetic apparatus for the production of electricity of high intensity | journal = Proceedings of the British Association for the Advancement of Science | volume = 7 | issue = | pages = 25 | publisher = BAAS | location = | date = 1838 | url = | accessdate = }} presented at meeting of September 1837 in Liverpool, England</ref><ref name="Page">{{cite book | last = Page | first = Charles Grafton | title = History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments | publisher = Intelligencer Printing House | date = 1867 | location = Washington, D.C. | pages = 26–27, 57 | url = http://books.google.com/books?id=lrzn9ZX79jAC&pg=PA26#v=onepage&q&f=false | doi = | id = | isbn = }}</ref> Johann Philipp Wagner (1839), and Christian Ernst Neeff (1847).<ref name="Britannica" /><ref name="Neeff">{{cite journal | last = Neeff | first = Christian Ernst | title = Ueber einen neuen Magnetelektromotor (On a new electromagnetic motor) | journal = Annalen der Physik und Chemie | volume = 46 | issue = | pages = 104–127 | publisher = | location = Berlin | date = 1839 | url = http://books.google.com/books?id=iBcAAAAAMAAJ&pg=PA104#v=onepage&q&f=false | issn = | doi = | id = | accessdate = February 14, 2013}}</ref><ref name="Neeff2">{{cite journal | last = Neeff | first = C. | title = Das Blitzrad, ein Apparat zu rasch abwechselnden galvanischen Schliessungen und Trennungen (The spark wheel, an apparatus for rapidly alternating closings and openings of galvanic circuits) | journal = Annalen der Physik und Chemie | volume = 36 | issue = | pages = 352–366 | publisher = | location = | date = 1835 | url = http://gallica.bnf.fr/ark:/12148/bpt6k15121m/f368.image | accessdate = February 14, 2013}} Description of Neeff and Wagner's earlier toothed wheel interrupter</ref> [[Hippolyte Fizeau]] (1853) introduced the use of the quenching capacitor.<ref name="Britannica" /><ref name="Fizeau">{{cite journal | last = Fizeau | first = H. | title = Note sur les machines électriques inductives et sur un moyen facile d'accroître leurs effets (Note on electric induction machines and on an easy way to increase their effects) | journal = Comptes rendus | volume = 36 | issue = | pages = 418–421 | publisher = Elsevier | location = | date = 1853 | url = http://gallica.bnf.fr/ark:/12148/bpt6k2993z/f422.image | accessdate = February 14, 2013}}</ref><ref>{{cite web|last=Severns|first=Rudy|title=History of soft switching, Part 2|work=Design Resource Center|publisher=Switching Power Magazine|url=http://www.switchingpowermagazine.com/downloads/Oct%2001%20soft.pdf|accessdate=2008-05-16}}</ref> [[Heinrich Ruhmkorff]] generated higher voltages by greatly increasing the length of the secondary,<ref name="Britannica" /> in some coils using 5 or 6 miles (10&nbsp;km) of wire, and produced sparks up to 16 inches. In the early 1850s, American inventor [[Edward Samuel Ritchie]] introduced the divided secondary construction to improve insulation.<ref>American Academy of Arts and Sciences, ''Proceedings of the American Academy of Arts and Sciences'', Vol. XXIII, May 1895 - May 1896, Boston: University Press, John Wilson and Son (1896), pp. 359-360</ref><ref>Page, Charles G., ''History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments'', Washington, D.C.: Intelligencer Printing House (1867), [http://books.google.com/books?id=lrzn9ZX79jAC&pg=PA104#v=onepage&q&f=false pp. 104-106]</ref> Callan's induction coil was named an [[List of IEEE milestones|IEEE Milestone]] in 2006.<ref>{{cite web |url=http://www.ieeeghn.org/wiki/index.php/Milestones:Callan%27s_Pioneering_Contributions_to_Electrical_Science_and_Technology,_1836 |title=Milestones:Callan's Pioneering Contributions to Electrical Science and Technology, 1836 |author= |work=IEEE Global History Network |publisher=IEEE |accessdate=26 July 2011}}</ref> Induction coils were used to provide high voltage for early [[gas discharge]] and [[Crookes tube]]s and other high voltage research. They were also used to provide entertainment (lighting [[Geissler tube]]s, for example) and to drive small "shocking coils", [[Tesla coil]]s and [[violet ray]] devices used in [[quack medicine]]. They were used by [[Heinrich Rudolf Hertz|Hertz]] to demonstrate the existence of electromagnetic waves, as predicted by [[James Clerk Maxwell|James Maxwell]] and by [[Oliver Lodge|Lodge]] and [[Guglielmo Marconi|Marconi]] in the first research into radio waves. Their largest industrial use was probably in early [[wireless telegraphy]] [[Spark-gap transmitter|spark-gap radio transmitters]] and to power early [[cold cathode]] [[x-ray tube]]s from the 1890s to the 1920s, after which they were supplanted in both these applications by AC [[transformer]]s and [[vacuum tube]]s. However their largest use was as the [[ignition coil]] or spark coil in the [[ignition system]] of [[internal combustion engine]]s, where they are still used, although the interrupter contacts are now replaced by [[Solid state (electronics)|solid state]] switches. A smaller version is used to trigger the [[xenon flash lamp|flash tube]]s used in cameras and [[strobe]] lights. ==See also== * [[Ignition coil]] * [[Trembler coil]] * [[Spark gap transmitter]] * [[Transformer]] * [[Tesla coil]] * [[Faraday's law of induction]] * [[Ignition system]] * [[Inductor]] * [[Magnetic field]] * [[Nicholas Callan]] ==Footnotes== {{reflist}} ==Further reading== * Norrie, H. S., "''Induction Coils: How to Make, Use, and Repair Them''". Norman H. Schneider, 1907, New York. 4th edition. * {{cite book|last=Collins|first=Archie F.|title=The Design and Construction of Induction Coils|date=1908|publisher=Munn & Co.|location=New York|url=http://books.google.com/?id=dJNPAAAAMAAJ&pg=PA98}} * {{cite book|last=Fleming|first=John Ambrose|date=1896|title=The Alternate Current Transformer in Theory and Practice, Vol.2|publisher=The Electrician Publishing Co.|url=http://books.google.com/?id=17sKAAAAIAAJ&pg=PA16}} Has detailed history of invention of induction coil ==External links== * [http://www.rmcybernetics.com/projects/DIY_Devices/homemade_ignition_coil_driver.htm Battery powered Driver circuit for Induction Coils] * [http://www.crtsite.com/page8.html The Cathode Ray Tube site] * {{Citation |first=F. H. |last=Newman |title=A New Form of Wehnelt Interrupter |journal=[[Proceedings of the Royal Society A]] |volume=99 |issue=699 |date=1 July 1921 |pages=324–330, plates 3 and 4 |url=http://ia600501.us.archive.org/25/items/philtrans03262334/03262334.pdf |publisher=Royal Society of London |doi=10.1098/rspa.1921.0045 |bibcode = 1921RSPSA..99..324N }} {{DEFAULTSORT:Induction Coil}} [[Category:Transformers (electrical)]] [[Category:Electrical breakdown]]'