Method for measuring the current on an electric conductor by means of the Faraday effect

Two lightwaves of a laser light source (3) having different frequencies ( omega 1, omega 2) are each supplied via a supplying optical fibre (10a, 10b) to an arrangement with polarisers (5, 6, 7, 8a) and a beam divider (4a). One wave leaving this arrangement consists of a left-hand and a right-hand circular polarised component having the first ( omega 1) and, respectively, the second frequency ( omega 2). It is conducted via an optical coil (2a) and supplied to an analyser (8b). The optical coil (2a) comprises an electrical conductor (1). The wave leaving the analyser (8b) is supplied to a phase comparison arrangement (13) via a first optical output fibre (10) and a photodetector (11a). The other wave, which leaves the arrangement with polarisation (5, 6, 7, 8a) and beam divider (4a), is directly supplied to the phase comparison arrangement (13) via a second optical output fibre (10c) and a photodetector (11b). The phase (2 theta ) between the intensities of the waves supplied to the phase comparison arrangement (13) is twice the rotation of the plane of polarisation of the light conducted through the optical coil (2a).