Experiments have been performed to investigate voltage scaling in a plasma opening switch (POS) as the radius of the cathode is varied. Two different opening switch configurations have been used. In one of these the anode and cathode are simple coaxes with constant radii. In the other, there is an increase in anode radius immediately downstream of the switch. It is found that for all the data the peak voltage of the POS follows a simple scaling law with cathode radius for short conduction times, but that as conduction time is increased the two sets of data diverge. The configuration with the increased anode radius shows a marked improvement in operating voltage at long conduction times as compared to the simple hardware, however, the data from each set of hardware continue to follow the derived scaling law for all conduction times observed. ©1995 American Institute of Physics.