SPARK COIL, OR RUHMKORFF COIL
The coil is an induction coil, which is a form of transformer designed to produce high voltage sparks across the spark gap above the coil. This principle is used in cars to produce the sparks in the spark plugs.
Ruhmkorff coils have been made that can produce sparks up to 1.2 metres long.
Heinrich Daniel Ruhmkorff developed the coil in the 1850s from designs of some earlier experimenters. It is another device operating on the principle of electro-magnetic induction.
Two separate coils are wound on to a magnetic iron core. One is a low voltage coil, the other a high voltage coil.
The low voltage coil has relatively few turns. It is connected to a DC source, typically a battery, by the two terminals at the left hand end of the machine.
The high voltage coil has very many turns. It is connected to the spark gap through the two terminals on the top of the machine
As the electro-magnetic induction effect of a transformer requires a change of current for the effect, the current to the low voltage coil is rapidly switched on and off by an interrupting device in the battery circuit.
A capacitor, usually fitted in the base of the machine, increases the effectiveness of the machine.
The induction coil has been used for a number of purposes. Early radio transmitters, called spark transmitters, used this device to produce the broadcast signal.