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1116 HANDBOOK OF PROJECTION FOR
To understand the action of the circuit shown in Fig. 406 we must first consider the arrangement. We have here fixed resistance A B connected across a 110-volt supply circuit. There is also a voltmeter, V, one terminal of which connects to resistance at A ; the other to sliding contact C.
Suppose we now move C along A R until it arrives at B. We then have for all practical purposes connected V directly across the 110-volt circuit. It will therefore register 110 volts. Suppose, however, we set C in the center of A B ; in other words, midway between A B. We then would get a reading of 110-4-2 = 55 volts, since at that point (the middle half) the original supply voltage (110) has been used up in forcing the current through the resistance of A. Similarly by making a connection one-third the distance from A, the voltmeter would read 110-4-3 = 36% volts. If the connection made one-tenth of the way from A, the reading would be 110 -J 10 = 11 volts.
Obviously, then, by moving C back and forth along resistance A we may alter the voltage applied to V through the entire range, from 110 volts down to zero. In other words, we can vary the power available at V from zero to maximum.
Now if for the 110-volt supply indicated in Fig. 406 we substitute the terminals of the motion picture projector film pick-up amplifier circuit, and if in place of voltmeter V we have the input terminals of the main amplifier, it is evident that by altering the position of C we will be able to supply to the main amplifier with all or with any fraction we may please of the power available from the pick-up, and naturally the volume obtained from the horns will be in exact proportion to the power