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REPRODUCING SYSTEMS
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In the latter the current consists totally of the electrons emitted from the cathode from the efTect of the light falling upon the sensitive surface, but in the former the cathode-emitted electrons ionize the inert gas, and, as a consequence, when a sufficient voltage is applied the current is amplified within the cell itself.
The amount of this amplification is limited by the ionization voltage, that is, the voltage at which a visible glow discharge takes place, as this effect is self-perpetuating and injurious to the cell.
The efTect of the illumination on the cell depends not only upon the material of which the cathode is made, but also upon the design of both the cathode and anode, as the photo-electric current depends upon the total illumination at any instant, and if the anode interferes with this illumination the current is reduced.
Two general types of construction are common: those with a central anode and those with a central cathode.
Figure 84 shows a central-anode cell, the type most generally used, with the cathode built in a semi-circular shape around the anode. The envelope is of glass or quartz, depending upon the metal used for the cathode and the source of illumination.
In an ideal cell, the photo-electric current is proportional to the total illumination but in practical applications of this principle the relationship is slightly non-linear because of the charging efTect of the glass wall, interference of the anode, reflection effects, and other obscure phenomena not yet fully understood.
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It is necessary that the imped ^10 ance looking into the output of the t 8 cell be held to a low value to I minimize this efTect, but the impedance must also be high enough to insure protection against glow discharge. Figure 85 shows the relative current plotted against illumination at different values of terminating resistances.
The photo-electric efTect of any certain material depends upon the wave length of the light rays to which the cathode is exposed. In motion picture work a cathode of
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— Reprinted by permission from "Electrical Engineer's Handbook," by Pender &" McIlwain, published by John Wiley & Sons, Inc.
Figure 85 — Current-illumination relation for gas-filled photo-electric cell with various series resistances.