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Publish On: 2018-12-11

Guy white

Total Post: 819

Question: Describe various types of electro emissions

Reply On: 2014-01-28

lala singh

Total Post: 22

ANS: Describe various types of electro emissions

Electrons may be obtained from atoms in all states of matter; solid liquid and gas. However, we shall here be concerned with four methods of liberating electrons from metal surfaces.

In field emission electrons are stripped from a metal surface by a high electrostatic field, of the order of millions of volts per centimeter. In the field-emission microscope surface structure of a pointed metal emitter may be made visible in an image produced on a fluorescent screen by electrons diverging from the tip. In a field ion microscope, Dr. E. W. Mu ̈ller has used the emission of positive ions from a needle like specimen to obtain image in which individual atoms can be distinguished.

In secondary emission, a metal is bombarded with high-speed particles which cause the ejection of electrons from the surface. Depending on the type and energy of incident particle, generally the arrival of each particle causes the emission of two or more electrons. This effect is utilized in the photoelectric multiplier tube which simplifies weak light signals.

It was shown earlier that in photoelectric emission, light or other radiation of suitable frequency falling on a surface cause emission of electrons. A photoelectric tube may most consist of a cathode having a photosensitive surface and an anode suitably arranged in a glass bulb, which is either evacuated or filled with an inert gas at low pressure. Common light-sensitive surface are potassium, cesium sodium, rubidium, and certain oxides. The photoelectric current from the cathode is proportional to the intensity of the incident radiation. We have seen that there are numerous applications of the photoelectric effect in reproduction of sound from motion-picture film, and in light operated relays and signaling devices.

In thermionic emission, electrons are “boiled” from a filament maintained at a high temperature. This effect was discovered by Thomas A. Edison during his development of light bulbs abs was applied by J.A Fleming, Lee De Forest, and others in a variety of electron tubes for the generation, amplification, and detection of signals in the form of electrical oscillations.   

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