By A. M. Howatson (Auth.)
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7. First ionization coefficients. breakdown value. Such a device is frequently made with cylindrical geometry, and the field is therefore not uniform. It is easy to show that the product ad in eqn. 1) must then be replaced by α άχ where rt and r2 are the radii of cathode and anode surfaces respectively. The ratio i/i0 is frequently called the multiplication factor or the electron multiplication. ) In a device like a photocell, it is found that there BREAKDOWN 59 exists a pressure for which the multiplication at fixed voltage is a maximum.
6. DIFFUSION It is well known that a gas which initially has a non-uniform concentration ultimately assumes a uniform distribution within its container if there is no appreciable field of force acting (in many laboratory applications the gravitational field of force may be ignored). In the same way, two initially separate gases become uniformly mixed. The process by which the condition of equilibrium is reached is diffusion. 1. The Diffusion Process Consider the concentration n of a simple gas in a container to vary, for any reason, in the x-direction only at some instant; there is then a net transfer of particles across any plane normal to x.
1), but the two are not identical since the electrons here have some distribution of energies and no single value of cross-section can apply. Considering the increase dn, over a distance dx, in the number of electrons per second crossing a plane distant x from the cathode, we may write dn = otndx Integrating from the cathode to this plane, we get n x «0 0 or, if a is independent of x, n = n0 ea* where n0 is the number of electrons per second leaving the BREAKDOWN 55 cathode. 1) where /0 is the electron current at the cathode and is dependent only on the photoelectric effect of external radiation.