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172
.MOTION PICTURE HANDBOOK
The Regulating Reactance, 21, Fig. 88, is simply a choke •coil with taps taken off at certain points along the winding so that the alternating current can be choked back or reduced to give the desired direct current in the projection arc. It gives very much the same effect as a resistance (rheostat) would, but with practically no waste of current. By using the various taps on the reactance almost any D. C. amperage can be obtained, within the capacity of the rectifier.
Shaking Magnet. — The shaking magnet, Fig. 86, is an electro-magnet used to rock the tube, which is necessary in starting, as will be explained later under "Theory."
Relay. — The relay, Fig. 86, is another electro-magnet with two contacts under its glass cover at the top. These open the shaking coil circuit as soon as the rectifier tube starts.
Theory. — The mathematical demonstration of the theory of the Mercury Arc Rectifier is rather complicated, but I will try to explain in as few words as possible the reasons for its action.
Fig. 87 shows a very elementary diagram of the rectifier connections, with all the different accessories, such as shaking magnet, relay, etc., omitted, leaving only the principal parts, rectifier tube, main reactance, and regulating reactance, all shown
diagrammatically, a n d with no reference to position on the panel with relation to one another. Each anode of the rectifier tube, it will be seen, is connected to a separate side of the alternating current supply; also through one-half of the main reactance to the negative side of ri the arc lamp.
The cathode connects to the positive side or upper carbon of the arc lamp. The small starting anode "C" is connected through the starting anode resistance to one side of the alternating current circuit, and is