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CURRENT RECTIFICATION 291
and require no lining up. They need no "warming up," but are ready as soon as the switch is closed.
(57) Substances other than copper oxide are sometimes used for the coating. Copper sulphide has been tried, and a number of combinations of metal and coating have been patented. Copper oxide remains the most effective for projection current rectifying purposes.
(58) At the present time copper-oxide rectifiers are made only for 3-phase current. They can be made in either single or 2-phase and doubtless will be if there is sufficient demand.
Fig. 741 is the circuit drawing of a three-phase, copper-oxide disc rectifier, essentially the same as that illustrated in Fig. 74H. Power supply to Fig. 741 is obtained from two separate sources, which are a threephase, 220-volt a.c. line and a single-phase, 110-volt a.c. line. The purpose of using two separate sources is to make possible control of this rectifier by the same 110volt a.c. switch that supplies power to the sound apparatus and to the projector motors. Through a relay built into Fig. 741 the same switch can be made to control the three-phase arc supply also, and hence to serve as a master switch for the entire projection room.
When this arrangement is followed the rectifier of 741, which has no tubes to watch or change and needs practically no attention, can be installed in the generator room, the cellar, or any other convenient place, and switched on in the morning and off at night by means of the 110-volt master switch.
There are two 110-volt circuits in Fig. 741. The first (from the input at the right of the drawing) merely runs through the fan motor, setting the fan in action as soon as the external master switch is closed. The second 110-volt circuit may be traced from the upper 110-volt wire to the mercoid switch shown just above the fan. This switch consists of a small glass tube, with two electrical contacts sealed into the glass, and a pool of mercury that closes the circuit between them when the glass tube is tipped to accumulate mercury in that location. When the tube is tipped the other way the mercury runs down to the