Edit filter log

'{{About|a specific type of [[transformer]]|the more general electrical component|Inductor}} An '''induction coil''' or "spark coil" ([[archaism|archaically]] known as an '''inductorium''' or '''Ruhmkorff coil''' after [[Heinrich Ruhmkorff]]) is a type of [[disruptive discharge]] [[coil]]. It is a type of electrical [[transformer]] used to produce high-voltage pulses from a low-voltage [[direct current]] (DC) supply. 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''. Developed beginning in 1836 by [[Nicholas Callan]] and others, the induction coil was the first type of transformer. They were widely used in [[x-ray machine]]s and [[spark-gap transmitter|spark-gap radio transmitter]]s from the 1880s to the 1920s. Today their only use is in physics education to demonstrate [[Faraday's law of induction|induction]], and as the [[ignition coil]]s in [[internal combustion engine]]s. 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. [[File:Induktionsapparat hg.jpg|thumb|270px|Antique induction coil used in schools, Bremerhaven, Germany]] ==How it works== [[File:Induction coil cutaway.png|thumb|270px|Induction coil showing construction, from 1920.]] An induction [[coil]] consists of two coils of insulated copper wire wound around a common [[magnetic core|iron core]]. One coil, called the ''[[primary winding]]'', is made from relatively few (tens or hundreds) turns of coarse wire. The other coil, the ''[[secondary winding]],'' typically consists of many (thousands) turns of fine wire. An [[electric current]] is passed through the primary, creating a [[magnetic field]]. 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 separating the secondary's output terminals. For this reason, induction coils were called spark coils. The size of induction coils was usually specified by the length of spark it could produce; an '8&nbsp;inch' (20&nbsp;cm) induction coil was one that could produce an 8&nbsp;inch arc. ===The interrupter=== [[Image:Induction coil waveforms.svg|thumb|left|Waveforms in the induction coil, demonstrating how the interrupter works. The <font color="blue">blue</font> trace, <font color="blue">''i₁''</font> is the current in the coil's primary winding. It is broken periodically by the vibrating contact of the interrupter. The voltage induced in the secondary, <font color="red">''v₂''</font> shown in <font color="red">red</font>, is proportional to the rate of change (slope) of the primary current. Both the "make" and "break" of the current induce pulses of voltage in the secondary, but the current change is much more abrupt on "break", and this generates the high voltage pulses produced by the coil.]] To operate the coil continuously, the DC supply current must be broken repeatedly to create the magnetic field changes needed for induction. Induction coils use a magnetically activated vibrating arm called an ''interrupter'' or ''break'' to rapidly connect and break the current flowing into the primary coil. The interrupters on small coils were mounted on the end of the coil next to the iron core. The magnetic field created by the current flowing in the primary attracts the interrupter's iron armature attached to a spring, breaking a pair of contacts in the primary circuit. When the magnetic field then collapses, the spring closes the contacts again, and the cycle repeats. 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. A "snubber" [[capacitor]] of 0.5 to 15 [[Microfarad|μF]] is used across the contacts to quench the arc on the 'break', which causes much faster switching and higher voltages. So the output [[waveform]] of an induction coil is a series of alternating positive and negative pulses, but with one polarity much larger than the other. ==Construction details== To prevent the high voltages generated in the coil from breaking down the thin [[Electrical insulation|insulation]] and [[Electrical 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. The secondary coil is wound in many thin flat pancake-shaped sections (called "pies"), connected in [[Series circuit|series]]. The primary coil is first wound on the iron core, and insulated from the secondary with a thick paper or rubber coating. Then each secondary subcoil is connected to the coil next to it, and slid onto the iron core, insulated from adjoining coils with varnished paper disks. The voltage developed in each subcoil isn't large enough to jump between the wires in the subcoil. 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 was 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. 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 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== 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 century. In powerful coils the high primary current created arcs at the interrupter contacts which quickly destroyed the contacts. 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 about 200 [[Hertz (unit)|Hz]], and the ones used in high-power coils only operated at 20 - 30 Hz. 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>{{cite book|last=Collins|first=Archie F.|title=The Design and Construction of Induction Coils|year=1908|publisher=Munn & Co.|location=New York|url=http://books.google.com/?id=dJNPAAAAMAAJ&pg=PA98}} p.98</ref> [[Leon Foucault]] and others developed interrupters consisting of an oscillating needle dipping into and out of a container of [[mercury (element)|mercury]]. The mercury was covered with a layer of spirits which extinguished the arc quickly, causing faster switching. In large coils these were often driven by a separate electromagnet or motor. The largest coils used either electrolytic or mercury turbine interrupters. The electrolytic or Wehnelt interrupter, invented by [[Arthur Wehnelt]] in 1899, consisted of a short [[platinum]] needle immersed in an [[electrolyte]] of [[sulfuric acid]], with the other side of the circuit connected to a lead plate [[electrode]]. When the primary current passed through it, 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. Mercury turbine interrupters had a [[Centrifugal force|centrifugal]] impeller which threw liquid [[mercury (element)|mercury]] on metal contacts. The result again was a higher rate of "breaking" and higher power output. == History == [[Image:Callan's first induction coil.jpg|thumb|upright=0.66|Callan's first induction coil.]] [[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|year=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>{{cite book|last=Fleming|first=John Ambrose|year=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}} p.16-18</ref><ref>http://www.nuim.ie/museum/ncallan.html</ref> and improved by [[William Sturgeon]] and [[Charles Grafton Page]]. The early coils had hand cranked interrupters, invented by Callan and [[Antoine Masson]]. The automatic 'hammer' interrupter was invented by C. E. Neef, P. Wagner, and J. W. M'Gauley. [[Hippolyte Fizeau]] introduced the use of the quenching capacitor.<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, in some coils using 5 or 6 miles (10&nbsp;km) of wire. In the early 1850s, after examining an example of a Ruhmkorff coil, which produced a small spark of around ''2 inches'' (50&nbsp;mm) when energized, American inventor [[Edward Samuel Ritchie]] perceived that it could be made more efficient and produce a stronger spark by redesigning and improving its secondary insulation. His own design divided the coil into sections, each properly insulated from each other. Ritchie's induction coil proved superior to other designs of the day, initially producing a spark of ''10 inches'' (25&nbsp;cm) in length; later versions could produce an electrical bolt ''24 inches'' (60&nbsp;cm) or longer in length.<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'', Boston: Harvard University, Intelligencer Printing house (1867), pp. 104-106</ref> The full story of Page's invention of the induction coil in its modern guise is told in Robert Post, "Physics, Patents, and Politics: A Biography of [[Charles Grafton Page]]" (Science History Publications, 1976. In 1857, one of Ritchie's induction coils was exhibited in [[Dublin]], Ireland at a conference of the [[British Association]],<ref>Rogers, W. B. (Prof.), ''Brief Account of the Construction and Effects of a very Powerful Induction Apparatus, devised by Mr. E.S. Ritchie, of Boston, United States'', British Association for the Advancement of Science, Report of the Annual Meeting (1858), p. 15</ref> and later at the [[University of Edinburgh]] in Scotland.<ref>American Academy, pp. 359-360</ref> Ruhmkorff himself purchased a Ritchie induction coil, utilizing its improvements in his own work.<ref>American Academy, pp. 359-360</ref><ref>Page, 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= |date= |work=IEEE Global History Network |publisher=IEEE |accessdate=26 July 2011}}</ref> [[File:Callans 1863 induction coil .jpg|thumb|250px|Callan's largest induction coil (Model of 1863), showing 'pancake' secondary construction. It was 42 inches (106 cm) long and could produce 15 inch (38 cm) sparks, corresponding to a potential of approximately 200,000 volts.]] 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. [[Image:Igncoil.jpg|thumb|left|Automobile [[ignition coil]], the largest remaining use for induction coils]] == Wireless charging == [[Toyota]]'s heavy duty division, [[Hino Motors]], is testing a new kind of [[hybrid electric vehicle]] without a plug (hybrid outboard chargeable vehicle). The energy in the batteries doesn't come from a plug and a charging point, but it comes from a wireless charging system built into the road. A series of induction coils built into the road resonate energy at certain frequency, like radio waves. The bus is able to capture those waves and store the energy in its batteries.<ref>http://www.ecogeek.org/content/view/1431/</ref> == See also == * [[Charging station]] * [[Ignition coil]] * [[Spark gap transmitter]] * [[Transformer]] * [[Tesla coil]] * [[Electromagnetism]] * [[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|year=1908|publisher=Munn & Co.|location=New York|url=http://books.google.com/?id=dJNPAAAAMAAJ&pg=PA98}} * {{cite book|last=Fleming|first=John Ambrose|year=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] {{DEFAULTSORT:Induction Coil}} [[Category:Transformers (electrical)]] [[Category:Electrical breakdown]] [[ca:Bobina de Ruhmkorff]] [[de:Funkeninduktor]] [[es:Bobina de Ruhmkorff]] [[fr:Bobine de Ruhmkorff]] [[it:Rocchetto di Ruhmkorff]] [[nl:Vonkinductor]] [[ja:誘導コイル]] [[pl:Cewka Ruhmkorffa]] [[pt:Bobina de indução]] [[ru:Катушка Румкорфа]] [[tk:Induktiwlik tegegi]]'