Toroidal Transformers, CT's and Rogowski Coils

A Rogowski coil, named after the German Physisct Walter Rogowski, is an electrical device for measuring alternating current (AC) or high-speed current pulses.    It consists of a evenly wound helical coil of wire with the lead from one end returning through the centre of the coil to the other end, so that both terminals are at the same end of the coil. The winding is done on a non-magnetic/non-metal former of constant cross sectional area. The whole assembly is then wrapped around the straight conductor whose current is to be measured. The speciality is there is no metal (iron) core.