Direct experimental measurements of electron diffusion coefficients $D$ in helium have been carried out at 300°K in the range of $0<~\frac{E}{N}<2.7\mathrm{}\times {10}^{-16\mathrm{}}$ V ${\mathrm{cm}}^2$ ($0<~\frac{E}{P_0}<10\mathrm{}$ V ${\mathrm{cm}}^{-1\mathrm{}}$ ${\mathrm{Torr}}^{-1\mathrm{}}$) by an improved electron-density sampling method. Absolute errors less than ± 0.5% are claimed in the range $0<~\frac{E}{N}<9\mathrm{}\times {10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$ and less than 1.5% in the range $9\times {10}^{-17\mathrm{}}<\frac{E}{N}<2.7\mathrm{}\times {10}^{-16\mathrm{}}$ V ${\mathrm{cm}}^2$.

The obtained values of $D$ are in agreement with those of Crompton, Elford, and Jory in the range of $2.124\times {10}^{-19\mathrm{}}<~\frac{E}{N}<~3.03\times {10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$ studied by these authors. Also measurements of $\alpha W$ (where $\alpha$ is Townsend's first coefficient and $W$ the drift velocity) have been performed in the range $9.03\times {10}^{-17\mathrm{}}<~\frac{E}{N}<~2.6\times {10}^{-16\mathrm{}}$ V ${\mathrm{cm}}^2$ (corresponding to $3.2<~\frac{E}{P_0}<~10$ V ${\mathrm{cm}}^{-1\mathrm{}}$ ${\mathrm{Torr}}^{-1\mathrm{}}$). Values of $\alpha$ with an accuracy evaluated at ± 10% are obtained by dividing $\alpha W$ by the $W$ values found in the literature. These $\alpha$ values are to be compared with those of the literature, which are dispersed by a factor of 3 in the above range.

• Received 11 September 1968

DOI:https://doi.org/10.1103/PhysRev.179.186