Abstract: 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.

Abstract: 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, and a test body (1) rotatably suspended within the gas sample chamber; means (10) for creating an inhomogeneous magnetic field within the gas sample chamber; means for detecting rotational motion of the test (body 1), comprising a compact optical system including a light source (12) and photodetectors (13) arranged to detect a light signal indicative of the rotational motion; and an actuation system (4) arranged to keep the test body (1) substantially at a null position determined by said optical system, wherein the optical system comprises at least one photodetector (13) positioned on either side of the plane which is normal to the mirror and parallel with the rotation axis of the test body, having the light source (12) positioned in the plane.