A compact resistance and voltage comparator employing an isolating potential circuit with mechanical choppers has proved to be a versatile and accurate laboratory tool. Voltages can be compared with an accuracy of 10^-8 V and resistors with an accuracy of 2 parts in 10⁶ (d.c. operation) or 2 parts in 10⁵ with automatic cancellation of thermal e.m.f. (square wave operation). When a d.c. amplifier and recorder are used, voltage and resistance differences may be recorded with an accuracy of about 2 parts in 10³.