Abstract: An apparatus for measuring changes in a variable interference section of an interferometer comprises a laser source for producing beams of the frequency n.sub.1 and the frequency n.sub.2, polarizing beam splitters for producing cross-polarized partial beams of frequency n.sub.1 or n.sub.2, modulators for displacing the frequency of one of the partial beams by frequency f.sub.1 or f.sub.2, a pair of deflecting mirrors in each case and polarizing beam splitters for combining the partial beam n.sub.1, n.sub.1 +f.sub.1, n.sub.2, n.sub.2 +f.sub.2. The apparatus further includes two photodetectors, upstream of which are arranged a polarizer, a Michelson interferometer, a non-polarizing beam splitter for splitting the partial beams in each case into a measuring light beam or a reference light beam. The reference light beam passes to an associated photodetector. The measuring light beam passes into the Michelson interferometer and then to the associated photodetector.

Abstract: A toroidal or similar body (1) of a non-magnetic, insulating material, preferably glass fibre-reinforced plastic or ceramic is provided with at least three recesses for receiving in each case a corresponding one magnetoresistive sensor. The associated measuring directions of said sensors are orthogonal to one another. For producing a magnetic auxiliary field applied to the sensors, a coil is arranged on the body. for increasing the accuracy and reliability of the measured result, the body is provided with at least one further recess for receiving a further sensor. The measuring direction of the further sensor differs from that of other sensors and forms preferably the same angle with the other measuring directions. The body can be in multipart form and is in particular made from two identical parts.The apparatus can be used in electronic compasses and for north finding devices.