Resistances of devices or samples of material can be measured accurately by placing the sample between conducting plates attached to an electrooptic crystal. The plates are then charged, and the voltage-induced birefringence in the crystal is monitored optically. In this way very small conduction currents in the sample can be monitored without introducing an additional current path between the electrodes. Resistances up to 10(16) ? have been measured using LiNbO(3) as the electrooptic crystal.