The Mobilities of Electrons in Helium

Mobility of electrons in helium was determined for fields up to 50 volts/cm and at pressures of 447, 189, 117 and 49 mm of Hg, using, as in previous work, the alternating current method with high frequency oscillations of sine wave form. The He was purified by prolonged circulation over charcoal in liquid air, maintained by a mercury vapor pump. Spectroscopic observation indicated the impurity to be about 1/10,000, chiefly H. The results for the mobility constant, reduced to 760 mm pressure and computed in the ordinary way are well represented by the equation $K^{\prime }={\left[\frac{11.0 {\mathrm{}\left(10\right)\mathrm{}}^8}{(2.45+E_0 \frac{760}{p})}\right]}^{\frac{1}{2}}$, where $E_0=\frac{V_0}{d}$ is the electric field as read on a static voltmeter. It has been pointed out that since $K$ is a function of $E_0$, a correction is necessary because of the use of a varying field in the measurements. It is shown that this correction may be made by multiplying the constants in the equation for $K^{\prime }$ by factors which depend upon the form of the equation. The corrected equation is $K={\left[\frac{7.57 {\mathrm{}\left(10\right)\mathrm{}}^8}{(1.56+E_0 \frac{760}{p})}\right]}^{\frac{1}{2}}$. For $E_0=0\mathrm{}$, this gives $K_0=22,000\mathrm{}$, a value close to that given by Townsend's equation. Hence the mean free path seems to have the value given by the Kinetic Theory. Compton's theory agrees well for $E_0$ above 20 volts/cm, but gives too low values for lower fields.