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This paper is an account of an experimental investigation of the motions of free electrons in air by the method developed by Townsend. An improved form of apparatus is described with the appropriate theory. The following parameters of the electronic motion were determined as functions of the ratioZ/p of the electric field strengthZ to the gas pressurep : Townsend’s energy factor kr the drift velocityW , the mean free path at unit pressureL and the mean proportion n of its energy lost in collisions with gas molecules. The experimental data are given in the form of tables and curves. The drift velocityW is found by a new procedure based on the Hall effect and by comparing the velocitiesW so obtained with the direct measurements ofW by Nielsen &amp; Bradbury it is seen that the velocities of agitation are distributed approximately according to Druyvesteyn’s law whenZ/p exceeds 0.5. Bailey’s factorG , which is of importance in ionospheric studies, is obtained from the experimental dependence of η onk r . Theoretical formulae are derived fork r andW in terms ofL, G andZ/p . The theory of the new method for measuringW is given in an appendix.

Experimental and theoretical studies of the behaviour of slow electrons in air. I