Drift Velocity of Electrons in Helium

The drift velocity of electrons in helium at 300°K has been measured for $\frac{E}{p}$ values between 4×${10}^{-4\mathrm{}}$ and 40 volt/cm-mm Hg. The data for $\frac{E}{p}<1\mathrm{}$ volt/cm-mm Hg were obtained from measurements of electron transit time in a modernized version of the double shutter tube developed by Bradbury and Nielsen. The data at high $\frac{E}{p}$ were obtained from microwave measurements of the electron density in a positive column of a low-pressure discharge. The measured drift velocities are in good agreement with previous results in the $\frac{E}{p}$ range from ${10}^{-1\mathrm{}}$ to 3 volt/cm-mm Hg. At $\frac{E}{p}$ less than 3×${10}^{-3\mathrm{}}$ volt/cm-mm Hg the electrons are essentially in thermal equilibrium with the gas. Margenau's theoretical expression for the drift velocity of electrons in a gas for which the cross section for momentum transfer is independent of electron energy is found to fit the data for $\frac{E}{p}<1\mathrm{}$ volt/cm-mm Hg to the accuracy of measurements. The cross section which gives the best fit of the theory to the data is 6×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$.