________________
of a wire, so that it does not obstruct the path of the light falling on the cathode. When light of frequency above the theshold 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 advantage of this type of photocell are that here 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 accurate 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 perssure of a few mm of mercury. When the potential difference between the electrodes exctrodes 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 reproduction 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. 3. It is used in a photographic camera for the automatic adjustment
of aperture. 4. It is used to compare the illuminating powers of two sources of
light and to measure the illumination of a surface. 5. It is used for automatic counting of the number of persons entering
a hall, a stadium, etc. 6. It is used for automatic switching of street lights and traffic signals. 7. It is used for raising a fire alarm in the event of accidental fire in
buildings, factories, etc. 8. It is used in burglar’s alarms for houses, banks and treasuries.
GERÎ YETI 37
– GJA, 2004
71
Jain Education International
For Private & Personal Use Only
www.jainelibrary.org
