An apparatus for the measurement of the magnetic susceptibility of a gas mixture comprises: a gas sample chamber (8) adapted to receive the gas mixture; a test body (1) rotatably suspended within the gas sample chamber; magnets (10) within the gas sample chamber; a compact optical system for detecting rotation of the test body (1) including a light source (12), photodetectors (13) arranged to detect a light signal, and a mirror (5) attached to the test body (1). An actuation system is arranged to keep the test body (1) substantially at a null position. The optical system comprises at least one photodetector positioned on each side of the light source. The light source is arranged to emit a light beam towards the test body. The photo detectors are arranged to each detect an edge of a light beam reflected from the mirror.