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to cells, or better still, light sensitive cells which are classfiied as follows :
No. 1. The photo electric.
No. 2. The actino electric.
No. 3. The becqurel voltic (photo voltic).
This comprises the entire range of photo cells that are in general use in sound work.
In returning to the first named photo cell, the “photo-electric,” it is seen that there are a great many types on the market that use as many kinds of photo-sensitive materials. The predominating minerals used are sodium, potassium, caesium. This by no means restricts the alakloid metals which are sensitive to light. We also have rubidium, barium, lithium, strontium, which react to light in an appreciable degree. These elements are among those that emit the photo electrons when a light ray strike? them. The photo electrons are emitted in streams or better, as a cloud fi’om the surface that the light ray impinges upon. The gas in many cells is to provide a stronger means for the polarizing current to pass through, thus realizing a higher output of the photo cell.
The gas is ionized (made conductive) by the photo electrons as they
speed to the anode as shown in Figure One. The gases vary almost as much as the photo elements used, not only in pressure and weight, but in characteristic molecular arrangement. The various gases which are most responsive to ionization are helium, neon and argon and Western Electric’s aroen, which is suspected of being a combination of neon and argon. These gases are termed “noble” gases by virtue of their ionization properties.
There are also marketed photo electric cells which are called hard cells or vacuum cells. These do not have gases in them depending mainly on the emission of photo electrons as a means of current flow.
In going a little further in the gas and polarizing voltage relationship it is noticed that in commercial photo electric cells there is a large variance of pressures and attendant voltages due to the type of materials used and cubic content of the containers or envelopes. In some of these cells the maximum voltages are less than 100 volts, while others use voltages in excess of 250 volts. In obtaining the maximum response to frequencies in sound work it is, up to a certain limit, which is governed by the amount and pressure and molecular arrangement of the gas to increase the polarizing voltage to obtain a greater degree of amplitude in the photo electric cell without sacrificing sensitivity.
The maximum voltages are held within limits necessitated by what is known as glow discharge, “similar to a neon tube glow caused by intense illumination and excessive voltages.” The voltage must be less than the point which causes this form of hysteresis in the photo electric cell, and is determined by the gas content.
The photo electric action takes place in a photo electric cell when a ray of light (constant or pulsating) enters the cell and impinges upon the cathode which carries the photo sensitive material. These elements upon receiving the light rays respond by emitting a stream of photo elec
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