By A. M. Howatson (Auth.)
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Extra resources for An Introduction to Gas Discharges
4. 13), their drift velocity due to an i>field or a concentration gradient is affected by a magnetic field. e. 29) when we consider diffusion alone and take E = 0. Without the term in B this equation would yield the velocity of diffusion as before, with a coefficient given by eqn. 21). The solution of eqn. 29) shows that the effect of B is twofold. e transverse to B corresponds to a coefficient * " - ÎTÎK <2 30) ' where coc is the electron cyclotron frequency eBjm and rm is l/vm. e has a component parallel to B the diffusion it produces is governed by D e only and is, as we should expect, unaffected by B.
B. Electronic Ionic Impact Phenomena. ). 1. THE TOWNSEND DISCHARGE ANY sample of gas under normal conditions can be expected to contain a number of ions and electrons. At ground level the atmosphere contains an average in the order of 1000 positive and negative ions per cm 3 due to ultraviolet and cosmic radiation and radioactivity. The rate of ionization which maintains this number is in the range 2-10 c m - 1 s _ 1 . If two electrodes are set up, they emit a few electrons from the same causes.
The work function φ is theoretically identical to that for thermionic emission but, again because of imperfections, in practice the photoelectric value is rather larger. The frequency given by is known as the threshold frequency. If the photon has a higher frequency, the excess energy goes partly into kinetic energy of the emitted electron and partly into collision heating of the surface during escape. For most metals, the threshold lies in the ultraviolet, but prepared surfaces using the alkali metals, such as caesium, can have a threshold well into the infrared.