English: Oudin coil therapy equipment used in the Victorian-era medical field called electrotherapy from about 1900 to 1930. The Oudin coil (right) is a spark-excited resonant transformer circuit similar to a Tesla coil, which generates very high voltage, low current, high frequency electricity. A handheld electrode attached by a wire to the coil's high voltage terminal at top produces streamer arcs which were applied to the patient's body to treat various medical conditions. The induction coil (center) produces a potential of 2 - 15 kV which powers the Oudin coil oscillator, which generates 200 - 1000 kV at a frequency of 200 kHz to 5 MHz. The switchboard (left) contains the interrupter power supply which produces the pulsing DC current which powers the primary of the induction coil. The couch (foreground) was called an "auto-condensation couch". Its metal back acted as a capacitor plate to apply the currents to the patient's entire body. The return path for the current was through the ground electrodes on the arms, which the patient would grip. High frequency therapy was not generally painful for the patient, because currents of this frequency do not cause the sensation of electric shock