English: A high voltage Tesla coil used for patient treatment in the Victorian era quack medical field of electrotherapy around 1907. This was the "Hercules" model manufactured by Frederick Finch Strong. It produced radio frequency voltages of several hundred thousand volts and low current levels at frequencies of around 1 MHz. A pointed electrode connected by a wire to the high voltage terminal of the coil was held by the physician, and the luminous sparklike brush discharge was played over parts of the patient's body to treat various medical conditions. This was not painful for the patient, because electric currents with frequencies over 10 kHz do not cause the sensation of electric shock.
The unit consists of a rotary spark gap (visible in center) that applies pulses of current from a high voltage supply transformer in the cabinet to the Tesla coil at top, which is attached to Leyden jar capacitors in the cabinet to make a resonant circuit. The device could be configured in several different ways; as a Tesla coil to produce "Tesla-Thomson currents", and as an Oudin coil to produce "Oudin currents" which were thought to have different effects. The caption read: "The author's "Hercules" high frequency apparatus"