An instrument for measuring the resistive and reactive components of eleccal circuits without physical contact or breaking the circuits. The instrument consists of a transmitter, a receiver, a sensor head, a power supply, and a display. The sensor head is constructed of two transformer cores split and hinged so that it can be closed around part of the circuit to be tested. One transformer core is driven by an oscillator and induces ac voltage in the test circuit. Current flow depends upon the electrical characteristics, admittance, of the test circuit. The other transformer core acts as a receiver and its output is an ac voltage proportional to the current flowing in the test circuit. The ac signal is detected using a synchronous detector with reference derived from the transmitter, either in phase to measure the resistive component, or with a 90.degree. phase shift to measure the reactive component.