The Tesla Transformer
Tesla described his coil as "a simpler device for the production of electric oscillations" for use in the design of high frequency machines.
In this device, a primary transformer coil with a few turns of wire is connected to a selected condenser (or capacitor) through a spark gap. When the condenser is supplied with electric current, it continuously charges up to reach the point where it achieves the selected breakdown voltage of the gap, and a spark results. At the moment of sparking, the condenser and primary coil are connected and form an oscillating circuit.
As the charge-to-spark process is rapidly repeated, the high energy pulsation in the primary coil induces voltage in the secondary transformer coil which has many turns of smaller wire. Settings and adjustments of each circuit control the oscillation frequencies of each circuit and optimum operation is achieved when the oscillating frequencies match, i.e. resonate. Then the oscillation in the second coil is multiplied, the coil produces high voltage, and strong sparks are emitted by the secondary terminal. With this output voltage reaching many million volts, some exceptional lightning-like discharges can be created.