A photoelectric cell is a device which converts light energy into electrical energy. It works on the principle of photoelectric effect. It consist of a semicylindrical photosensitive metal plate C (emitter) and a wire loop A (collector) supported in an evacuated glass or quartz bulb.It is connected to the external circuit having a high tension battery and a microammeter (LA) as shown in Fig. (17.7).
When light of suitable wavelength falls on the emitter C photoelectrons are emitted. These photoelectrons are attracted towards collector A. A small photoelectric current produced in the circuit is recorded by microammeter connected in the circuit. If the intensity of incident radiation is increased, the rate of photoelectric emission increase causing an increase in the current. Thus current is Thus directly proportional to the intensity of incident radiation.
Applications of photoelectric cell:
i) Exposure meter: An exposure meter is used along with camera to determine the correct time of exposure for having photograph. It consists of a photoelectric cell, a battery and microammeter joined in series. When it is directed towards the object to be photographed, the light enters the photoelectric cell and sets up a current in the circuit. The extent of deflection of the pointer in the microammeter is directly proportional to the intensity of light. Generally the microammeter is calibrated to give the exposure time directly.
(ii) Burglar alarm: In this application, a photoelectric cell is used to activate relay mechanism. The light from an infrared lamp falls on the photosensitive surface of
photoelectric cell. The infrared beam is invisible and when some one crosses the beam, the light is cut-off and photoelectric current stops. This operates. the relay circuit and starts ringing the bell.
(iii) Sound reproduction from motion pictures: Light from a powerful lamp passes throûgh the sound, track at the edge.of the film and falls on photosensitive surface of photoelectric cell. The photoelectric current varies according to the intensity of light transmitted by the recorded sound track. The varying current is amplified and fed to the loudspeaker. The loudspeaker reproduces the sound recorded on the sound track.
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