Types PG-1 and 2 have all units-exciter lamp rheostats, ammeters and relaysmounted in the amplifier rack, the only control on the projector being the fader actuating switch.

One more switching device is used in connection with the above mentioned controls. This is the "Film-Disc" switch, illustrated in Figure 76, used on Types PG-1, 2, 3, 4, 6, 7 and 8. The function of this is merely to change one side of the output from the film or disc circuit, so that one or the other is always connected to the input of the amplifier. Since one side of the circuits from film and disc outputs is a common lead, only a single-pole switch is necessary. One such switch is provided for each projector. As mentioned before, this is mounted on the sound head on Types PG-10 and 13. All other equipments are provided with

a special switch box, which serves as a terminal box as well. It contains terminals for both sound head and turntable output circuits and necessary resistors and potentiometers for balancing film and disc output, so that each will be equal on the same projector. One form of this box is illustrated in the figure. This type is placed on the front wall of the projection room, near the operating side of the projector it controls. Another type is mounted on the sound head on top of the photo-cell housing. Both serve the same purpose and have similar types of switches. These are off in the center position. On Types PG-10 and 13 this switch is mounted in the photo-cell housing on the sound head, and is similar in purpose to the others, although different in appearance. It is a standard type of tumbler switch.

112. Rheostat and Potentiometer Connections.-Rheostats and potentiometers, as was mentioned in section 14, are nothing more than wires of relatively high resistance, much higher than those used ordinarily in electrical circuits, so made and connected in a circuit as to vary the amount of current passing through it.

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