File:Mercury turbine interrupter.png

English: A mercury turbine interrupter, an obsolete electric device used with large induction coils around 1900 to repeatedly interrupt the primary current to the coil, to create the magnetic flux changes necessary to generate its high voltage output. Small, low power induction coils use a vibrating arm interrupter on the coil to break the primary current. However these were inadequate for the large coils used to power x-ray machines and spark gap radio transmitters up to the 1920s. The high primary currents (up to 30 A) created sparks at the interrupter contacts which quickly destroyed them, and vibrating interrupters were not capable of interruption rates over about 50 breaks per second. Therefore mercury turbines were used on large coils.

The mercury turbine interrupter, invented around 1899, consisted of a glass tank (left) partly filled with the liquid metal mercury (omitted in this picture). The electric motor (right) drove a pump in the tank which sprayed a solid stream of mercury at the rotating sawtooth-shaped contacts (visible in tank) The mercury nozzle was connected to one side of the primary circuit, while the contact wheel was connected to the other. When the stream hit a tooth it completed the primary circuit, producing magnetic flux in the coil. When the stream fell between the teeth it broke the primary circuit, causing the magnetic field in the coil to collapse, inducing a high voltage pulse in the secondary of the coil. The advantage of the turbine was that it could handle very heavy currents, could produce interruption rates of up to 10,000 breaks per second, and the frequency and "dwell time" could be conveniently adjusted. The frequency of the breaks was controlled by varying the speed of the motor, while the "dwell time", the percentage of a cycle the circuit was closed, could be varied by raising or lowering the mercury nozzle. Lowering the nozzle caused the stream to hit the tooth at a lower level, where it was narrower, reducing the time the primary circuit is closed.