A method of determining the magnitude of an exceptionally large current in a conductor (4) which comprises measuring the angle of rotation of polarized light (6, 7, 8) from a light source (1, 2) in a magnetic field created by the current with a glass fiber (3) sensor composed of a fluoride glass having a low Verdet constant and a photoelastic coefficient not over about 0.25 (nm/cm)/(kg/cm ).