Abstract: In-service verification/continuity testing apparatus and method for testing the continuity integrity of wiring installed to an in-use operational circuit. The apparatus measures the voltage difference created by injecting two equal but opposite insignificant currents through a test path. A current loop is created by placing an isolated ground, which is referenced to unique detection circuitry provided in the testing apparatus. An accurate voltage differential calculation is accomplished by using a sample and hold circuit in parallel with switching the injected currents. Biasing the detection circuitry using two identical but inverse voltages with a reference to the isolated ground allows the detection circuit to subtract out extraneous currents flowing through the wiring path due to external voltage and current sources.