Low-current ignitors

SLEPIAN AND LUDWIG´S discovery of the ignitron¹ principle of initiating the cathode spot of an arc has been followed by intensive investigation of the factors affecting the design and operation of the ignitron. Many of the design features of the ignitron follow conventional mercury-arc-rectifier practice. The distinguishing feature of the ignitron is the ignitor. An ignitor is an electrode of high resistivity material which is partially immersed in the cathode mercury. On sending a sufficiently large current through the ignitor into the mercury, a cathode spot forms upon the mercury at the ignitor mercury junction. The current to accomplish this varies in an inverse manner with the resistivity of the ignitor so that it is necessary to use relatively high resistivity material for practical ignitors. In practice the ignitor is connected to a suitable source of power with some means of switching so that the arc may be started at the proper time during the interval when the ignitron anode voltage is positive. The most generally used circuit uses a thyratron for controlling the ignitor current as shown in figure 1.