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518 RICHARDSON'S BLUEBOOK OF PROJECTION
Varieties of Photo-Cells
Most modern photo-electric cells are gas-filled because the presence of gas increases the emission of such cells in precisely the same way that gas increases the emission of a rectifier tube filament. (See Page 508.) The gas used in photo-cells, however, is not mercury vapor, but one of the rare gases, usually argon.
(17) Photo-conductive, as distinct from photo-electric, cells are used in a few sound systems. They consist merely of small pieces of the metal selenium, suitably mounted in a convenient holder. This metal has the property of acting as insulator in the dark and as a conductor when illuminated. It is believed that light causes an internal emission of negatrons from its atoms, while in darkness these negatrons cling to their atoms and repel any free negatrons attempting to flow through the metal as part of a current. (See Page 432.) A strip of this metal can readily be substituted for the cell shown in Figure 120, with simple circuit modifications for retaining impedance match.
Photo-Electric Cell Coupling
(18) Figure 120 shows one common method of "coupling" — that is, wiring — the photo-electric cell to the switches, volume controls and amplifiers in which the pulsations of its direct current are controlled and amplified. In Figure 120 that current merely flows through the primary of a step-down transformer. The result is a flow of alternating current in the secondary winding that reproduces all the fluctuations of the pulsating direct current in the primary. But the voltage and amperage are altered. The voltage is reduced to 1/1 2th, and the amperage multiplied nearly 12 times. The sound current drawn from secondary of the transformer in Figure 120 passes through a fader before entering the system amplifier. The fader is essentially a double-throw, double-pole switch that connects the system amplifier with sound current from either projector. In some systems the fader serves as a volume control as well as a