The Turn Of The Century Electrotherapy Museum http://www.electrotherapymuseum.com High Frequency Variometer Coils Experimental Wireless Coils, Diathermy Coils, Tesla/Oudin Coils Left:  High Frequency Tesla / Oudin Coil; Right:  Diathermy / Thermo-Penetration Coil Alternately:  Left:  Resonator, Right:  Oscillator.  In this way the intensity of the Tesla circuit can be varied by swinging the angle one way or the other without breaking the circuit.  Tangential wire for the output - made with silicone and wire-wrap! Alternately still the coils can be demonstrated in a flat position, one coil being used to detune the other! Appearance of coils folded together.  Maximum output when used as an Oscillator / Resonator.