Patent ID: 8847579

Claim:
An apparatus for the measurement of the magnetic susceptibility of a gas mixture, comprising: a gas sample chamber ( 8 ) adapted to receive the gas mixture, and a test body ( 1 ) rotatably suspended within the gas sample chamber; magnets ( 10 ) for creating an inhomogeneous magnetic field within the gas sample chamber; a compact optical system for detecting rotational motion of the test body ( 1 ), the compact optical system including a light source ( 12 ), photodetectors ( 13 ) arranged to detect a light signal indicative of the rotational motion, and a mirror ( 5 ) attached to the test body ( 1 ) and arranged perpendicular to a main optical axis of the light source's emitted light beam; and an actuation system 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 positioned on each of two opposing sides of the light source, wherein the light source is arranged to emit a light beam towards the test body, and the photodetectors are arranged to each detect an edge of a light beam reflected from the mirror.