Abstract: An electrical testing instrument including an amplifier circuit having a pair of inputs, one comprising a probe and the other being adapted for manual engagement by the user for completing an input circuit through the body of the user. The circuit has a battery and a current-responsive indicator in its output circuit to provide a signal when current flows in the input circuit to provide a critical forward biasing of the amplifier. The battery is connected to furnish such input current when the input terminals are connected in a closed circuit having a resistance of at least approximately the resistance of the user's body. The amplifier circuit is further arranged that when the probe is contacted with a live AC power line and the manually engageable input terminal is so manually engaged, an operating indicating current will also be present in the output circuit.