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532 RICHARDSON'S BLUEBOOK OF PROJECTION
cal with that connecting the left-hand side of the potentiometer to Terminal No. 2. Upper Terminal No. 1 runs to Point O; lower Terminal No. 1 runs to Point 12 of the right-hand half of the potentiometer.
The reader may care to trace out the Disc input connections of this circuit. No. 1 disc input wiring is shown ; No. 2 disc input wiring is obviously the identical circuit applied to the opposite side of the potentiometer.
A number of control cabinet circuits similar to this one are equipped with supplementary key switches that do not alter the essential simplicity of the device, but add to the time required to trace out the drawings. One such switch may remove the potentiometer from the circuit entirely and substitute a resistance equal to that of a medium setting. This is for emergency, in case of trouble with the potentiometer resistors. Another such key is often provided to allow for the use of three projectors. This key is so wired that either No. 1 input or No. 2 input, as desired, may be used to take care of No. 3 projector. None of these circuits can be understood at a glance, but none of them will present any particular difficulty to any projectionist who is willing to take ten or fifteen minutes to trace their wiring step by step. Such analysis should, obviously, be made by every projectionist long before trouble occurs and compels him to attempt it under the pressure of an emergency.
Switching and NonSynchronous Sound
(36) Such control cabinets as that shown in Figure 125 are not normally used with non-synchronous sound. In theatres that use announcing microphones or nonsynchronous phonographs the output of Figure 125 would run to a small switching panel mounted with or near the main amplifier, not to that amplifier direct. By means of that switching panel the amplifier system can be connected either with the source of synchronous sound (the output of Figure 125) or with the non-sync source, as desired. The wiring of such switching panels is in the same class with that of Figure 125, in that it contains