It is a device to produce an almost unidirectional high voltage by using the principle of mutual induction.
Construction- It consists of a primary coil P of a few turns of thick copper wound over a core of soft iron. Carefully insulated from the primary is the secondary coil S consisting of a large number of turns of fine insulated copper wire. The terminals of the secondary are brought to the two points D, D.
In series with the primary is a make-and-break arrangement whose working is explained below:
Working- The current from the battery B passes through the commutator K and to the primary coil P through the screw contact at P, P’. The core is, thus, magnetized and the soft iron strip H is attracted towards the core, breaking the circuit at P, P’. The primary current then ceases, the core becomes demagnetized and the strip H falls back completing the circuit again. The same process is repeated over and over again.
Why the voltage is unidirectional- Voltage is induced in the secondary both at the time of make as well as the time of break of the primary circuit. However, as the inductance of the primary is quite high, the current rises slowly and the induced voltage at make is negligibly small compared to the induced voltage at the break- at break current falls rapidly, hence a very high voltage is induced. Hence the secondary voltage is almost unidirectional.
Function of Capacitor- A capacitor is usually placed across the gap P, P’ to reduce the chances of spark at P, P’. The capacitor also helps to increase the secondary voltage at the break by its charging and discharging action.