Hook-on Meter

an instrument for measuring electric voltages and currents in lines carrying heavy currents and high voltages, without making an electric contact with the current-carrying wires (busses). The head of a hook-on meter is constructed in the form of a split core made from a ferromagnetic material, with an induction coil mounted on it. The instrument is equipped with insulated grips or a handle and with a spring mechanism, making it similar in appearance to pliers.

To make a measurement the split core, together with the induction coil, is clamped around the current-carrying wire or bus, thus forming an instrument transformer; the wire acts as the primary winding, and the induction coil acts as the secondary winding. An AC pointer-type meter is connected to the induction coil.