Induction coil

thumb|Antique induction coil used in schools from around 1900, Bremerhaven, Germany

An induction coil or "spark coil" (archaically known as an inductorium or Ruhmkorff coil 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 coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter. spark-gap radio transmitters, One coil, called the primary winding (P), is made from relatively few (tens or hundreds) turns of coarse wire.

Curves supplied by a 1984 reference agree closely with those values.

Interrupter

To operate the coil continually, the direct current must be repeatedly connected and disconnected to create the magnetic field changes needed for induction. To prevent this, a quenching capacitor (C) of 0.5 to 15 μF is connected across the primary coil to slow the rise in the voltage after a break. The capacitor and primary winding together form a tuned circuit, so on break, a damped sinusoidal wave of current flows in the primary and likewise induces a damped wave in the secondary. As a result, the high-voltage output consists of a series of damped waves.

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. In one widely used technique, the secondary coil is wound in many thin flat pancake-shaped sections (called "pies"), connected in series. Léon Foucault and others developed interrupters consisting of an oscillating needle dipping into and out of a container of mercury.

History

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. The induction coil was invented by the American physician Charles Grafton Page in 1836 and independently by Irish scientist and Catholic priest Nicholas Callan in the same year at the St. Patrick's College, Maynooth and improved by William Sturgeon. The early coils had hand cranked interrupters, invented by Callan and Antoine Philibert Masson (1837). The automatic 'hammer' interrupter was invented by Rev. Prof. James William MacGauley (1838) of Dublin, Ireland, Johann Philipp Wagner (1839), and Christian Ernst Neeff (1847). Hippolyte Fizeau (1853) introduced the use of the quenching capacitor. Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, Jonathan Nash Hearder worked on induction coils. Callan's induction coil was named an IEEE Milestone in 2006.

Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and other high voltage 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 Clerk Maxwell and by Lodge and Marconi in the first research into radio waves. Their largest industrial use was probably in early wireless telegraphy spark-gap radio transmitters and to power early cold cathode x-ray tubes from the 1890s to the 1920s, after which they were supplanted in both these applications by AC transformers and 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 tubes used in cameras and strobe lights.

Footnotes

External links

Battery powered Driver circuit for Induction Coils
The Cathode Ray Tube site
Relay Technical Information See section "Contact Protection – Counter EMF".
Capacitive Discharge Ignition vs Magnetic Discharge Ignition: Ignition System Options for the TR4A See figure 9 for actual discharge.