Abstract: An electrical current measuring device, for measuring the current in an electrical conductor (11), includes first and second measurement channels. Each of these channels includes a sensor (12) adapted to be located adjacent the conductor (11) and coupled via optical fibers (14,16) to source circuitry (18) and detector circuitry (20). The sensor (12) incorporates a material (22) which exhibits the Faraday magneto-optic effect and the source circuitry (18) is arranged to deliver an unpolarized light signal which is subsequently polarized by a first polarizer (23) to enable a linearly polarized light signal to be incident to the material (22). The quantum of rotation in the plane of polarization imposed on the signal emerging from the material (22) is monitored via a second polarizer (24) by the detector circuitry (20) as a measure of the electrical current flowing in the conductor (11).