The motion of electrons in gases

Abstract

1.The methods of investigating the motion of negative ions in gases at low pressure that have been explained in some previous papers may be extended to cases in which larger variations are made in the electric force and pressure. In order to find the kinetic energy of the motion of agitation of the ions, the velocity in the direction of an electric force, and the value of e/m for different forces and pressures, it is necessary to investigate experimentally two properties which are characteristic of the motion of electrons. These are the abnormal lateral diffusion of a stream of ions moving in a uniform electric field, and the deflection of the stream produced by a small transverse magnetic force. In the previous experiments the two phenomena were investigated separately and in each case with apparatus which gave satisfactory results when small electric forces were used and the pressures were limited to a certain range. In order to investigate the motion under larger forces an apparatus of more suitable dimensions was constructed, by means of which both the required sets of experiments may be made. 2. The negative action of ultra-violet light on the plate A, fig. 1, and after traversing the distance from A to B some of the ions passed through a narrow slit S, 2 mm. wide and 15 mm. long, in the centre of the metal sheet B. The electric force was in the same direction on the two sides of B, so that the ions, after passing through the slit, continue their motion towards the plane electrodes C, which were parallel to the plane of B. The electrodes C wT ere 4 cm. from B, and three flat rings, Ri, R2, £3, 7 cm. internal diameter, were fixed at distances of 1, 2, and 3 cm. respectively from the plane of the electrode C. A separate connection for each ring and for the plates A and B was brought out through a large ebonite plug fitted in the brass cover of the apparatus, and was maintained at a potential proportional to the distance of the corresponding ring, or plate, from the electrodes C. The stream of ions ------R* ------Ra -R, C, C£ Cj F ig.

Transport Coefficients for Low Energy Electrons in Crossed Electric and Magnetic Fields

“…The method used by Townsend (Townsend and Tizard 1913) for measuring drift velocities may be understood by reference to Figure 1. For a given value of gas pressure p and electric field strength E, the magnetic field B at right angles to E and parallel to the transverse cut in the receiving electrode was adjusted until equal currents were received by the two sections A and C. By assuming that the motion of the centre of the stream was along the resultant of the forces Ee and Be W, Townsend obtained the following relation between the drift velocity W, the electric and magnetic field strengths, and the angle (J which is determined by the apparatus:…”

“…Measurements ofthe deflection of electron swarms, drifting in steady d.c. electric fields, produced by a magnetic field at right angles to the electric field date back to the pioneering work of J. S. Townsend (Townsend and Tizard 1913). Notwithstanding the revived interest in the measurement of transport coefficients for low energy electrons drifting and diffusing through gases at pressures ranging from several torr to several hundred torr (McDaniel 1964, and references therein) little has been done to add to this early work of Townsend and its extension by his colleagues to a number of other gases (Healey and Reed 1941, and references therein;Townsend 1948).…”

The Diffusion of Electrons in Dry, Carbon Dioxide Free Air

“…In comparing the results of these various investigations, and also in attempting to correlate the behaviour of electrons in air with that in oxygen and nitrogen, it is of interest to have available reliable data for the variation with E Ip (where E = electric field in V Icm and P = gas pressure in torr) of Townsend's energy factor kl for electrons in dry air which is free from carbon ruoxide and other condensable impurities. At present the only data available for Ejp > 20 are those of Townsend and Tizard (1913) while for Ejp < 20 additional measurements have been made by Bailey (1925) and in this laboratory by Crompton,. Huxley, and Sutton (1953) …”

Mechanisms of detection and imaging in the ESEM

“…All other parameters are directly available here and the fraction of SE moving in the v o b e between E and A can be readily found. The above formula was derived for slow electrons, but Townsend & Tizard (1913) have shown that, if an avalanche multiplication takes place, the relative fraction of electrons arriving at E out of the total number of electrons in the avalanche is still R. The average displacement of electrons upon arrival at the top electrodes is rD, an evaluation of which together with other detailed theoretical analysis is presented elsewhere (Danilatos, 1990). The above equation is of great practical importance in our case.…”