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SOURCES OF SOUND CURRENT 519
changeover. In such cases it is not built in the form of a switch, but of a double potentiometer, as explained on Page 530.
A second method of photo-cell coupling is shown in Figure 121. This method is used in connection with an amplifier mounted on or near the projector, and connected to the photo-cell by a very short wire. It deserves exceptionally careful study because the identical circuit is also used in amplifiers for "coupling," or wiring together, amplifying tubes.
CATHODE
CONDENSER.
AMPLIFIER. INPUT
D.C.+
Fig. 121.— Photo-cell coupling. (Condenser-resistance coupling).
In Figure 121 negatrons from the d. c. source travel to the cathode of the cell, are emitted under the influence of light, and cross the cell to the anode. Thence they move through Resistor A to the positive side of the d. c. source, thereby completing their circuit.
But a parallel circuit can be traced from the d. c. negative terminal through the common ground, thence upward through Resistor B, then through the condenser and downward through Resistor A to the positive d. c. terminal. Since this circuit contains a condenser in series with it, it cannot carry smooth d. c. but only the ripple, or a. c. component, created by the photo-cell action. (See Page 467.) This ripple or a. c. component is precisely the part that corresponds to the sound to be reproduced, and by the circuit of Figure 121 it has now been transferred to Resistor B, which looks into the amplifier. The alternating voltage drop across Resistor B is wired directly to the input terminals of the amplifier.