Dielectric Breakdown of Liquid Helium

The dielectric strength of liquid ${\mathrm{He}}^4$ under its saturated vapor pressure has been determined from 1.2 to 4.2°K. With $\frac{3}{8}$-inch diameter spherical steel electrodes 0.15 mm apart the average breakdown field, ${\overline{E}}_b$, is approximately 1 Mv/cm, nearly independent of temperature. At spacings of 0.5 and 1 mm, ${\overline{E}}_b$ is lower, and appears to decrease with decreasing temperature below 2.5°K. The mechanism of breakdown is not clear, but field emission of electrons from the cathode is probably involved, because the average breakdown voltage ${\overline{V}}_b$, obtained with a point and plane electrode system when the point is negative, is about half that obtained when the point is positive, both above and below the $\lambda$-point. Pre-breakdown currents were never detected, and were probably less than ${10}^{-13\mathrm{}}$ amp, implying an electrical resistivity for liquid helium of ${10}^{18}$ ohm-cm.