English: Effluvation treatment of knee around 1920 using an Oudin coil, a transformer invented in 1893 by French physician Paul Marie Oudin, similar to a Tesla coil which produced very high voltage radio frequency electricity. This was used to treat many diseases in the Victorian era medical field of electrotherapy. Here a multipointed electrode attached to the coil (left) produces a luminous brush discharge (effluve). This is held by the doctor and played over the patient's knee. The patient holds a metal pipe electrode which is attached to the top of the coil to serve as the return path for the current. The Oudin coil produced a potential of 200,000 to a million volts. This was not painful for the patient since the frequency of the current was so high; around 100 kHz to 2 MHz. Alternating current with frequencies above around 10 kHz do not cause the sensation of electric shock when applied to the body. High frequency electrotherapy caused a heating of tissues, and evolved into the modern field of diathermy