Townsend Theory Research Articles

The Mobility of Electrons in Pure Nitrogen

Mobility of electrons in pure nitrogen was determined, using the Rutherford alternating potential method, for pressures ranging from $75 \mathrm{to} 600$ mm., for frequencies ranging from $7,000 \mathrm{to} 150,000$ cycles (obtained from an audion oscillating circuit), and with electric fields of from 10 to 100 volts/cm. Reduced to atmospheric pressure the mobility found is of the order of 10,000\ifmmode\cdot\else\textperiodcentered\fi{} cm. sec.,/many times the highest value previously obtained, and it was observed to vary with pressure and electric field according to the equation $K=\frac{571,000}{(21+760 \frac{V}{\mathrm{pd}})}$, where $\frac{V}{d}$ is in volts/cm. and $p$ in mm. of Hg. A discussion of the possible sources of error shows that none can be responsible for this variation with $\frac{V}{\mathrm{pd}}$, and the form of the mobility curves confirms this variation. A theoretical interpretation of these results on the basis of the Townsend equation for electron mobility leads to the conclusions: that if the Townsend theory is correct (1) either the energy lost at each impact of an electron with a nitrogen molecule or the mean free path must be a function of $\frac{V}{\mathrm{pd}}$, and (2) in either case the mean free path for velocities of the normal agitation must be about 22 times the mean free path of the gas molecules instead of $4 \sqrt{2}$ times, as given by the kinetic theory.

The Effect of Field Direction on Magneto-Resistance

Variation of Magneto-resistance Effect with Direction of the Magnetic Field.\char22{}(1) Theoretical discussion. The fact that in general the resistance of a conductor differs slightly according to whether the magnetic field is transverse or longitudinal, may be due to two factors. First, there may be a direct effect of the field on the motion of the electrons, as predicted by the electron theories of J. J. Thomson and others. But the author shows that if Townsend's method of developing the electron theory of conduction is adopted, the variation of resistance comes out zero. Second, the effect of the magnetic field may vary with the arrangement of crystals in the conductor, and therefore with the position of the conductor if the crystal structure is anisotropic. (2) Experiments with cost bismuth, pressed graphite and rolled cadmium are described in which a small bar or sheet of the material was placed in each of three mutually perpendicular positions in a field of 7,000 or 8,000 gausses, and resistance measurements were made for various positions of the rotatable Weiss magnet. For bismuth and cadmium the variations found are not symmetrical around the current direction and are evidently due chiefly to the crystal structure. For graphite the variations when the field was rotated in the plane of the sheet were within the experimental error, therefore both factors were inappreciable. These results are not conclusive but they suggest that the Townsend theory is correct. If so this magneto-resistance effect may be completely explained by assuming a change in the number and mean free period of the conducting electrons which depends not only on the magnetic field but on its direction with reference to the crystal axes.

The electric strength of air. — VII

In view of the variation among the values obtained by different observers, this paper aims to make a careful determination of corona-forming voltages for alternating and for positive and negative continuous voltages in the same apparatus and under the same conditions. Within the range of wire diameters used, corona appears at a lower value when the wire is positive than when it is negative, although the two curves converge for increasing diameters. The maximum excess of negative over positive, as observed, was 6.3 per cent. The values with alternating voltage coincide with those of negative continuous voltage. Positive continuous voltage therefore forms corona at the lowest value. The observations on the negative corona give values higher than any heretofore obtained. Other experiments are described, giving qualitative indications of the correctness of Townsend's theory of ionization by collision.

The Electric Strength of Air.-VII

In view of the variation among the values obtained by different observers, this paper aims to make a careful determination of corona-forming voltages for alternating and for positive and negative continuous voltages in the same apparatus and under the same conditions. Within the range of wire diameters used, corona appears at a lower value when the wire is positive than when it is negative, although the two curves converge for increasing diameters. The maximum excess of negative over positive, as observed, was 6.3 per cent. The values with alternating voltage coincide with those of negative continuous voltage. Positive continuous voltage there-fore forms corona at the lowest value. The observations on the negative corona give values higher than any heretofore obtained. Other experiments are described, giving qualitative indications of the correctness of Townsend's theory of ionization by collision.

XXXVI. Ionization by collision

1. 1–3. The physical assumptions made in Townsend's theory of ionization by collision are analysed and discussed. It is pointed out that one of them is to the effect that the negative ion does not ...