English: Long wave diathermy apparatus developed around 1908 by German physician Karl Franz Nagelschmidt the founder of diathermy, with which he performed the first experiments on patients. Diathermy is the therapeutic application of high frequency currents to the body for deep tissue heating. The device consisted of a spark discharge Tesla coil which produced damped waves of several thousand volts at a frequency of 1 to 3 MHz. At the top right of the unit is the series spark gap which excited oscillations in the primary circuit, at top left is a hot-wire ammeter which monitored the current passing through the patient. The current was applied to the patient's body with two metal plate electrodes resting on brine-soaked pads; the current heated the tissue between the electrodes. This was not uncomfortable for the patient because alternating currents over 10 kHz do not cause the physiological sensation of electric shock. The different electrodes are visible on the shelf under the unit. Around 1930 long wave diathermy was superseded by short wave diathermy which used frequencies of 10 to 100 MHz produced by vacuum tube oscillators.