A device used to measure common-mode current flowing along a wire or collection of wires. It consists of a ferrite core that clamps around the wire(s) and a coil wrapped around the core. A magnetic field is induced around the core as specified by Ampere’s law; the magnetic field induces a voltage around the coil as a result of Faraday’s law (see electromagnetic induction) and this voltage is then measured. From a calibration using a standard current, the measured voltage can be used to calculate the current flowing along the wire(s).