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MANAGERS AND PROJECTIONISTS 567
Mercury Arc Rectifier
THE mercury arc rectifier is a device marketed by two manufacturers, the General Electric Company and the Westinghouse Electric and Manufacturing Company. Its purpose is to change alternating current of standard line voltage to direct current at arc voltage, the reduction in pressure being accomplished by means of an auto-transformer, which is an integral part of the machine.
Kindly understand that we have, to some extent, sacrificed strict technical correctness to "understandableness" in the following:
PRINCIPLE OF OPERATION.— The mercury arc rectifier consists essentially of a sealed glass bulb, from which the air has been exhausted, provided with four terminals, A, Al, B and C, Fig. 183. Within this tube is a quantity of mercury, the purpose of which will be explained further on. The two upper terminals A. Al. Fig. 183, are of graphite or other suitable material, the two lower ones B, are of mercury, C, Fig. 183, which is the smaller of the two, being what is known as a "starting terminal." When the bulb is in a vertical position the pools of mercury in terminals B and C are separated, but when the tube is tilted or rocked sidewise to the left, they are brought temporarily into contact, for the purpose of putting the tube into action.
When in its active state the vacuum bulb contains vapor of mercury, which is a conductor of electricity only under certain conditions. Current will readily pass from graphite terminals, A or Al, Fig. 183, into the mercury vapor, and when the arc is burning and the circuit thus completed, will pass from it into mercury terminal B, and thus on through the arc.
Alternating current, however, changes its direction many times a second, and when the direction of flow seeks to reverse itself and pass from the mercury to the graphite terminals, the mercury vapor offers sufficient resistance to prevent it. The mercury vapor thus acts as a check valve, permitting the current to pass from the graphite into the mercury vapor, into the mercury and on throught the arc, but preventing it