Abstract: Measurement of the current flowing in the rotor of an electrical machine and especially an a.c. exciter with external poles for excitation of a synchronous machine by way of a rectifier mounted on the rotor is accomplished without the use of sliprings by means of at least one pair of contra-wound coils extending in the axial direction of the rotor, the coils being connected in series and mounted in a slotted support of insulation material secured to the underface of a pole shoe in the gap between shoe and a toothed rotor. A voltage proportional to the rotor current is induced in the pair of coils by the stray flux generated by the current carrying conductors in the rotor slots. The width of each coil approximates the width of one tooth on the rotor and the spacing between the axes of the coils approximates the size of the rotor slot pitch.