Method of measuring alternating currents without disturbing the conducting circuit

It is shown that a thin toroidal `worm¿ coil which forms a closed loop around a current-carrying conductor can be arranged to give an output voltage proportional to the time rate of change of current. Such a probe can be used to measure currents of any frequency in situations where space is limited without disturbing the conducting circuit in any way. It can therefore not only be a viable alternative to a current transformer, but also be used where measurement is not possible by conventional means.