The characters of practicable magnetic potentiometers have not been rigorously proved. This paper deduced a mathematical description of mutual inductance of the cylindricalmagneticpotentiometerwithfinite ts crosssection and verifies that M is independent of the position with respect to the primary circuit and is unaffected by stray fields of current not linked with them. Using these features, a novel device is designed to measure ACcurrent of the measuring range 0~4 000 A. The experimental result shows the measuring error is less than 1%.The device holds various advantages: such as the fixed number of ts of the primary winding, low cost, small size and weight. This paper pointed out that if the device is used to measure the nonsinusoidal signal, an integrator can increase the accuracy.rn