The Tesla Coil

When a charged capacitor of capacitance C discharges through a coil of inductance L and low resistance, the discharge is oscillatory. The frequency of oscillation is is 1/2π√ (LC).  The frequency will be high if the values of L and C are small. Now if this inductance is made the primary of an air core step-up transformer, the voltage induced at the secondary is very high and the frequency of alternation being the same as of the primary circuit will also be very high. Such an arrangement makes the Tesla coil.

P is an inductance containing a few turns of a thick copper wire forming the primary of an air core step up transformer. The secondary coil S contains any turns of thin copper wire and is an narrower coil instead P. The primary is connected in series with a capacitor C to the spark gap G of an induction coil. One end of both the primary and secondary coil is earthed. The other end of secondary has such a high voltage that it gives spark several cms long. Sparks from this coil can be taken by the naked hand without being hurt.

The emf is high. The frequency of alternation is also very high. High frequency ac travels only over the surface of conductors. Hence the current due to the spark does not pass through the body.