Induction Coil - Mercury and Electrolytic Interrupters

Mercury and Electrolytic Interrupters

Although modern induction coils used for educational purposes all use the vibrating arm 'hammer' type interrupter described above, these were inadequate for powering the large induction coils used in spark-gap radio transmitters and x-ray machines around the turn of the century. In powerful coils the high primary current created arcs at the interrupter contacts which quickly destroyed the contacts. Also, since each "break" produces a pulse of voltage from the coil, the more breaks per second the greater the power output. Hammer interrupters were not capable of interruption rates over about 200 Hz, and the ones used in high-power coils only operated at 20 - 30 Hz.

Therefore much research went into improving interrupters, and improved designs were used in high power coils, with the hammer interrupters only used on small coils under 8" sparks. Léon Foucault and others developed interrupters consisting of an oscillating needle dipping into and out of a container of mercury. The mercury was covered with a layer of spirits which extinguished the arc quickly, causing faster switching. In large coils these were often driven by a separate electromagnet or motor.

The largest coils used either electrolytic or mercury turbine interrupters. The electrolytic or Wehnelt interrupter, invented by Arthur Wehnelt in 1899, consisted of a short platinum needle anode immersed in an electrolyte of dilute sulfuric acid, with the other side of the circuit connected to a lead plate cathode electrode. When the primary current passed through it, gas bubbles formed on the needle which repeatedly broke the circuit. This resulted in a primary current broken randomly at rates up to 2000 breaks per second. Mercury turbine interrupters had a centrifugal impeller which threw liquid mercury on metal contacts. They had interruption rates up to 10,000 breaks per second, and were the most widely used type of interrupter in commercial wireless stations.

Induction coil by Heinrich Ruhmkorff. In addition to the hammer interrupter (right), it had a mercury interrupter by Fizeau (left) that could be adjusted to change the dwell time. One of the largest coils ever constructed, built in 1877 by Alfred Apps. Wound with 280 miles of wire, could produce a 42 in. (106 cm) spark, corresponding to roughly 1,200,000 volts. Modern automobile ignition coil, the largest remaining use for induction coils