Continuous current control, so familiar in the operation of high-vacuum tubes, has not been possible, except under special circumstances, for gas tubes. Even current interruption has been awkward, except for low currents, for the usual manner of interrupting the current is to decrease the anode potential to zero. The time to switch off the gas tube has been of the order of the deionization time for the gas employed. Now a method is developed for switching off mercury-pool arcs using a third electrode. There is no interference with the main power circuit and, in fact, the potential on the anode causing the electric field aids the dispersal of the charge carriers when the arc has been interrupted. The switching-off time is much decreased because this anode-to-cathode voltage sweeps all the charge carriers out of the tube. Switching off is effected by passing a reverse current of equal or greater magnitude than the arc cathode current through the tube for a time long enough to interrupt the cathode spot. A technical difficulty arises in that the third electrode introducing the reverse current has to have an already formed or an easily formed cathode spot since this third electrode is a cold cathode. Many methods for forming the cathode spot are discussed. The method finally used is probably not the best one but it has the virtue of being easily effected. There appears to be no limit as to the current that can be interrupted if the spot-forming mechanism is altered. Energy used in not an important factor. The amount varies with the time to switch off and does not influence the actual switching process. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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