Abstract
This paper describes measurements of conduction currents in both neat and commercial insulating liquids. Conduction in thermodynamic equilibrium has been measured by applying a low ac voltage in a uniform field. High-field pre-discharge currents have been measured in a needle-to-plane configuration stressed with respectively high voltage ac and steep step high voltage pulses. There was significantly increased conduction for high field compared to low field conditions. Notably, measurements at high fields in needle-plane configurations indicate space charge limitation of currents with polarity differences. The high field conduction exhibited considerable variation across different liquids. At high applied fields, electronic processes are probably governing the charge formation. Simulations indicate that space charges may reduce the local field in front of needle tips at time scales relevant for propagating streamers.