The ‘Pansanitor’ (1928)

Electrical muscle stimulation machines offer relief from aches and pains, muscle tension, neuralgia and lots of other miscellaneous ailments, using either low- or high-frequency alternating currents. Generating these currents is bread-and-butter stuff using modern semiconductor technology, but how was it done eighty years ago, in a time when transistors, let alone integrated circuits and microcontrollers, were not exactly off-the-shelf components?