Effect of humid air on power loss in pure and impure paraffin oil

Measurements were made over the frequency range 50 c/s¿30 Mc/s on a very pure paraffinic oil, and on the same oil containing a few parts per thousand of an ester of the type used in the plasticizer trade. Neither displayed a measurable power loss in the low-frequency region, the oil containing the ester showing the usual dipole loss in the megacycle region. Treatment with humid air had no effect on the pure oil, but produced a serious low-frequency power loss in the oil containing an ester. This loss was proportional to the square root of the ester content and increased rapidly as the voltage was raised from 50 to 250 volts on a film 0.025 cm thick (2 to 10 kV/cm). The conductivity corresponding to this power loss is roughly constant from 100 to 1000 c/s and falls rapidly to a low value around 3000 c/s.