A companion paper by Gardner and Hawke described a microwave phase shifter, with constant output, capable of rapid phase control by the bias voltage applied to varactor diodes. This paper describes the application of this device to measure phase shifts in a cw microwave system by a null method. If phase and amplitude variations are not too fast, standard oscilloscope components can provide most of the required circuitry. Examples are given, including the measurement of 0.003° phase shift at 9 Gc in a plasma experiment.