coil connected to electrodes at different places on the patient's body. The high frequency currents could produce heat in the patient's body, similar to the operation of a microwave oven; this was called Diathermy (Behary). Other names were also used such as Thermo-Faradism and Thermo-Penetration depending on whether it was used for therapy or for destructive effects in surgery (Behary) Oudin resonator - French physician Paul Marie Oudin in 1893 modified the D'Arsonval circuit by adding a second large "resonator" coil made of many turns of fine wire to a tap on the small D'Arsonval coil. By adjusting the tap point he could bring the second coil to resonance with the tuned circuit, generating higher voltages at lower currents, similar to the Tesla circuit. In early "Oudin resonator" apparatus the two coils are separate and not magnetically coupled. Later Oudin found that the efficiency could be increased by magnetically coupling the coils, making the small coil the primary winding for the big coil. The Oudin resonator generated very high voltage, low current electricity. It was a "unipolar" circuit, with one end grounded and a single output terminal. Tesla-Thomson apparatus - Serbian-American engineer Nikola Tesla pioneered the field of high-voltage RF resonant transformers, inventing the Tesla coil circuit in 1891. He and Elihu Thomson experimented with applying high voltage to their bodies. Although Tesla invented many different resonant transformer circuits, the electrotherapy circuit that bears his name was a "bipolar" circuit consisting of a spark-excited tuned primary coil of few turns encircling the center of a symmetrical bipolar secondary coil of many turns of fine wire, with output terminals on each end. The D'Arsonval and Oudin apparatus became popular in Europe, while the Tesla-Thompson apparatus was mostly used in America. During the first decades of the 20th century there was a rivalry between these camps, and debate in the medical literature as to whether "Tesla currents" or "Oudin currents" were better for various conditions. By 1920 it was realized that the currents were very similar. Since the circuits were so similar, medical suppliers sold combination "high frequency" units that could be set up for Tesla, D'Arsonval, or Oudin therapy, often also combined with Rontgen ray (X-ray) After Oudin combined the primary and "resonator" coil together on the same form, making them an air-core autotransformer, the only significant difference between the Tesla and Oudin apparatus was that the medical Tesla coil was