How and Why? - Tesla Coil

(518) Lionel Stevens, Sidney, Australia. inquires:

Q. 1. What is the principle of operation of the so-called Tesla coil?

A. 1. The principle upon which the Tesla coil is based consists in introducing high frequency currents from the discharge of the condenser into the winding of a special transformer; the condenser being charged by means of an induction coil or by a transformer operating on low frequency and high tension. The apparatus may be connected in two ways, both pointed out by Tesla in 1891. In the first arrangement, Fig. 1, the circuits (1) and (2) represent either an induction coil or a commercial high voltage transformer. The coil charges two condensers, C and C₁, which are connected to the primary (3) of a high frequency transformer. A spark gap (AB) is connected to the terminals of the circuit (2).

As the current in the secondary (2) increases, the condensers C and C₁ are charged until they reach a potential sufficient to discharge over the gap (AB). This spark causes a sudden variation in the potential of the condensers and a current of extremely high frequency is set up in the primary (3). This high frequency current is a result of the oscillatory nature of the discharge across the gap (AB). Currents of the same high frequency but very much higher voltage are set up in the secondary (4) of the Tesla coil. The operation of the second type of Tesla circuit shown in Fig. 2 is similar to that outlined above.