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An induction coil or "spark coil" (archaically known as a 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 DC supply. To create the flux changes necessary to induce voltage in the secondary, the DC current in the primary is repeatedly interrupted by a vibrating mechanical contact called an interrupter. Developed beginning in 1836 by Nicholas Callan and others, the induction coil was the first type of transformer.

The term 'induction coil' is also used for a coil carrying high-frequency AC producing eddy currents to heat objects placed in the interior of the coil, in induction heating or zone melting equipment.
Antique induction coil used in schools, Bremerhaven, Germany

How it works

Induction coil showing construction

An induction coil consists of two coils of insulated copper wire wound around a common iron core. One coil, called the primary winding, is made from relatively few (tens or hundreds) of 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 volts. This voltage is often sufficient to cause an electrical discharge, or 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 inch' induction coil was one that could produce an 8 inch arc.

The interrupter

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 attracted an 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.

Although opposite potentials are induced in the secondary when the interrupter 'breaks' the circuit and 'closes' the circuit, the current change is much more abrupt when the interrupter 'breaks', so the pulse of voltage induced in the secondary at 'break' is much larger. A 'snubber' capacitor 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.

Mercury and electrolytic interrupters

The small 'hammer' interrupters described above were used on coils creating up to 8 inch (~120 kV) sparks. Larger coils used motor-driven interrupters. The largest coils, used in radio transmitters, used either electrolytic or mercury turbine 'breaks'.

Construction details

To prevent the high voltages generated in the coil from breaking down the thin insulation and 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. 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 coated with an insulating layer like paraffin, connected to the coil next to it, and slid onto the iron core, insulated from adjoining coils with 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 prevent eddy currents, which flow perpendicular to the magnetic axis, and cause energy losses, the iron core is made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically.


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.

Michael Faraday discovered the principle of induction, Faraday's induction law, in 1831 and did the first experiments with induction between coils of wire. The induction coil was invented by the Irish scientist Nicholas Callan in 1836 at the St. Patrick's College, Maynoothmarker 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. Neeff, P. Wagner, and J. W. M'Gauley. Hippolyte Fizeau introduced the use of the quenching capacitor. Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, in some coils using 5 or 6 miles of wire. In the early 1850s, after examining an example of a Ruhmkorff coil, which produced a small spark of around 2 inches (50 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 cm) in length; later versions could produce an electrical bolt 24 inches (61 cm) or longer in length. 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 Dublinmarker, Ireland at a conference of the British Association, and later at the University of Edinburgh in Scotland. Ruhmkorff himself purchased a Ritchie induction coil, utilizing its improvements in his own work.

Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and for X-ray research. They were also used to provide entertainment (lighting Geissler tubes, for example) and to drive small "shocking coils", Tesla coils and violet ray devices used in quack medicine. They were used by Hertz to demonstrate the existence of electromagnetic waves, as predicted by James Maxwell and by Tesla and Marconi in the first research into radio waves. Their largest industrial use was probably in early wireless telegraphy radio transmitters and to power cold cathode x-ray tubes. By about 1920 they were supplanted in both these applications by vacuum tubes. However their largest use was as the ignition coil or spark coil in the ignition system of internal combustion engines, where they are still used, although the interrupter contacts are now replaced by solid state switches. A smaller version is used to trigger the flash tube used in cameras and strobe lights.

Wireless charging

Toyota's heavy duty division, Hino Motorsmarker, 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.

Early patents

  • The induction-coil, instead of being made movable upon the magnet
  • This compound coil is made like any ordinary induction-coil
  • The inner end of the induction-coil are surrounded by the prime coil
  • The induction-coil consists of a metallic conductor, copper is generally preferred
  • Energizing the primary wire of the induction-coil, the iron core becomes magnetized
  • Making use of an induction-coil
  • a split-coil improvement (1903).
  • Induction coil comprising a soft iron core (Mar 5, 1913)

See also


  1. p.98
  2. p.16-18
  4. 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
  5. 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
  6. 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
  7. American Academy, pp. 359-360
  8. American Academy, pp. 359-360
  9. Page, pp. 104-106

Further reading

  • Norrie, H. S., "Induction Coils: How to Make, Use, and Repair Them". Norman H. Schneider, 1907, New York. 4th edition.
  • Has detailed history of invention of induction coil

External links

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