Tesla's Oscillator/Earthquake machine
Tesla's oscillator is purely mechanical. Steam would be forced into the oscillator, and exit through a series of ports, the net effect of which was to cause the armature to vibrate at high speed, within its casing. The casing was by necessity very strong, as temperatures due to pressure heating in the upper chamber exceeded 200 degrees, and the pressure reached 400psi. Other versions of the machine were created, designed to produce electrical power, both alternating and direct, (without the need for rectifiers). Another variation used electromagnets to control the frequency of the piston's oscillation.