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SOURCES OF SOUND CURRENT 531
casing. The four terminals immediately below "G" are marked "Film 1 and 2." The output leads from the film reproducing system of each projector are wired to these t \v< ) sets of terminals. About an inch below these are two terminals marked "Output." Through these this changeover device connects with the main amplifier. Below these, again, are two pairs of terminals for the synchronous disc input.
Let us return to the top left of this drawing, to the film input from No. 2 projector. The upper wire of this pair is brown and may be traced to Point O of the potentiometer. The lower wire, brown-red, runs to point 12 of the left-hand side of the potentiometer. Thus the entire resistance of the potentiometer is connected in parallel across the input from film reproducing system No. 2. Sound current coming from No. 2 projector, originating in its photocell, completes its circuit through the lefthand half of this potentiometer, and a correspondingalternating voltage drop exists across that resistance.
Now, looking at the two output terminals, we find that the upper peg, connected to the slate wire, also makes contact with Point O of the potentiometer, while the lower output terminal runs through the slate-red wire to the rotating element of the potentiometer, and its sliding contact. By means of this sliding contact (indicated by the double arrowhead which in the drawing connects the rotating element with Point O) any desired percentage of the voltage drop across the left-hand side of the potentiometer may be connected with the output terminals, and hence with the system amplifier. Volume from No. 2 projector is controlled by turning the central knob and thus changing the position of the sliding contact.
To change over, all that is necessary is to swing the central knob so that the sliding contact operates on the right-hand side of the potentiometer. Doing so disconnects the upper Output terminal from Projector No. 2, and places it in contact with Projector No. 1. The wiring from Film Terminals No. 1 to the right-hand side of the potentiometer is omitted from the drawing because it is an unnecessary complication. This wiring is identi