An ac self‐balancing potentiometric photoconductive circuit has been developed and employed to measure resistive ratios. It includes a cadmium sulfide photoconductive cell and an incadescent lamp in an ac negative feedback configuration. The tested ratio is measured by incorporating the photocell in a dc circuit and monitoring the resulting current which is proportional to the tested ratio. Two specific examples are described, one of which is a precision circuit that was used to construct an inductive conductivity meter for oceanographic applications. By properly selecting the photocell type and increasing the open loop gain, a 0.1% accuracy (of full scale) may be obtained with a relatively simple circuit. The circuit is applicable for signal conditioning of resistive transducers such as platinum thermometers and strain gauges. It may be particularly useful in cases where a dc output signal is preferred while the resistive device must be driven by an ac signal.