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26. Satish K. Gupta. op. cit., pp. 1092, 1093—
"Photoelectric Cells A photoelectric cell is an arrangment which converts light energy into electrical energy. Photoelectric cells are of the following three types : 1. Photoemissive cells, 2. Photovoltaic cells, 3. Photoconductive cells. A photoemissive cells may be of vacuum type or gas filled type. Let us discuss the working of a photoemissive cell. Photoemissive cell. It consists of two electrodes, a cathode C and an anode A enclosed in a highly evacuated glass bulb. The cathode C is a semi-cylindrical plate coated with a photosensitive material, such as a layer of cesium deposited on silver oxide. The anode A is in the form of a wire, so that it does not obstruct the path of the light falling on the cathode. When light of frequency above the threshold frequency for the cathode surface is incident on the cathode, photoelectrons are emitted. If a potential difference of about 10 V is applied between the mode and cathode, the photoelectrons are attracted towards the anode and the microammeter connected in the circuit will record the current. The chief advantagen of this type of photocell are that there is no timelag between the incident light and the emission of photoelectrons and that the photoelectric current is proportional to the intensity of light. The cell is extremely acccurate in response. Hence, it is used in television and photometry. The current may be increased by a factor of about 5 by filling the tube with an inert gas at a pressure of a few mm of mercury. When the potential difference between the electrodes exceeds the ionisation potential of the gas, the emitted photoelectrons ionise the gas atoms and now a larger current flows. Applications of Photoelectric Cells Photoelectric cells have a variety of application in industries and the daily life. A few important applications of photoelectric cells are as given below: 1. It is used for the reporduction of sound from the sound track.
recorded on one edge of the cinema films. 2. It is used in a television studio to convert the light and shade
of the object into electric currents for transmission of picture.
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