Abstract: Gas testing apparatus comprises a gas chamber wherein a test body surrounded by a single turn electrical conductor is suspended by a metal strip in a non-uniform and non-symmetrical magnetic field which is formulated by especially shaped pole pieces which could, in one form, by permanent magnets. The test body is deflected from the initial position it takes when a reference gas is in the cell, by the admittance of a gas mixture of which is paramagnetic gas is one constituent. The deflection is sensed by an optical system comprising a light source, a mirror on the test body and a pair of photocells. The output of the photocells feeds an amplifier system, which in turn feeds the single turn coil by a current of an adequate value such that it stops the test body from moving from its initial position and hence is proportional to the partial pressure of the paramagnetic gas in the sample.