Induction Coil

There are numerous forms of induction coil, all principally based upon the transformer of Michael faraday. The most common form consists of a core of thin iron wires. Around this is wrapped a winding of a hundred or so turns of fairly thick wire, known as the primary. When a current is passed through this coil, the core becomes magnetised. The magnetism attracts a piece of iron mounted on a spring, this in turn breaks the circuit to the primary. When the current ceases the iron moves away from the core, re-making the contact. Various forms of this interrupter were developed during the Victorian era. Most commonly the contacts used were platinum or silver, however in the example shown above the interrupter (M) consists of a moving contact and a pool of mercury.

When the primary circuit is broken its inductance causes a reverse or back emf, which may be ten times that of the applied voltage. This may cause an arc to form, slowing down the rate of change of the primary current. In order to reduce this arcing, a capacitor is commonly connected across the contacts. This may often be found in the base of the instrument.

The secondary of the induction coil consists of thousands of turns of fine insulated wire. A high voltage, typically of the order of tens of thousands of volts is induced in the secondary. Frequently a spark gap is connected across the secondary. It is in fact inadvisable to operate a coil without an appropriate spark gap, particularly if it is an old coil, as arcing can occur between turns of the secondary, damaging the insulation.