<PICT:0761967/III/1> <PICT:0761967/III/2> <PICT:0761967/III/3> <PICT:0761967/III/4> In a paramagnetic gas analyzer of the type wherein a gas current flows past an electric resistance heating means under the action of a magnetic field and the temperature difference between two portions of the gas is measured by electrically heated measuring resistances connected to the arms of a Wheatstone bridge, or of the type where the electric resistance heating means is itself employed as the measuring resistance of the bridge circuit, the supply of electric current employed for heating the resistance heating means is separate and distinct from the supply of current fed to the Wheatstone bridge for measuring purposes, the first mentioned supply being a low frequency A.C. current or a direct current and the second mentioned supply being an audio-frequency alternating current. As diagrammatically shown in Fig. 1 a Wheatstone bridge 3 containing in two of its arms the electrically heated measuring resistances of an analyser cell (described below in Fig. 4) is connected to an oscillator 1 through a triode 2 whereby adio-frequency current is fed to the bridge, the other diagonal of the bridge being connected to a step-up transformer 4 connecting the bridge to an amplifier 5, a detector 6 and a measuring instrument or relay 7. The analyser cell is shown in Fig. 4 where a gas current entering at 30 divides into two portions 31, 32 which re-unite and flow out at 33. A central arm 34 is provided with electric heating resistance 36 and is subjected to the action of a magnetic field indicated at 35 by dotted lines. Electrical measuring resistances 37, 37a at each end of the channel 34 are connected to adjacent arms of a Wheatstone bridge circuit. Under the action of the magnetic field 35 a gas mixture containing a paramagnetic gas for example oxygen or nitric oxide is diverted from A to B by the magnetic field and thus enters tube 34 where it loses its magnetism under the heating effect of heater 36 and continues to flow towards B whereby the thermal balance between resistances 37 and 37a is disturbed, the unbalance being a function of the paramagnetic gas content. In such a bridge the heating resistance 36 is fed with direct or low frequency alternating current and the measuring resistances 37 and 37a are fed with audio-frequency A.C. Alternatively the heating resistor 36 is omitted and the heating is effected by superimposing on the audio-frequency current flowing to elements 37 and 37a, a low frequency heating current. The measuring resistances 37, 37a may also be replaced by thermistors. The circuit is shown in Fig. 2 where the secondary of a transformer 10, the primary of which is connected to mains supply through voltage regulator 9, is connected to a rectifier 11 followed by a smoothing and filtering circuit 12 and gas discharge valve 13. The stabilized feed thus produced is fed to a phase-shift or resistance-capacitance type oscillator comprising an oscillator pentode 14 and tuned circuit 15 containing a variable potentiometer P1 to adjust the oscillator to the desired value. The oscillator is followed by a triode 16 to match the impedence of the oscillator with that of the measuring bridge and a potentiometer P2 to adjust the amplitude of the emitted undamped wave. The secondary winding of transformer 17 is connected across one diagonal of the Wheatstone bridge 18 comprising four resistances E1, E2 (corresponding to 37, 37a of Fig. 4) and R1 and R2. A potentiometer P3 enables the zero point of the instrument to be adjusted. The Wheatstone bridge output is fed through a transformer 19 to an amplifier of the resistance-coupled type comprising an amplifier pentode 20 and resistances 21 whereby part of the amplified alternating voltage at the anode of the pentode is applied to the grid of a cathode-follower triode 22, the anode of which is connected to high tension positive. The cathode of valve 22 follows the variations of alternating voltage and transmits them through a capacitance coupling 23 to a selective network 24 whence they are returned to the amplified input. The selective network 24 introduces negative feed back covering the full frequency range with the exception of the fundamental frequency, in practice a band width of about 10 cycles, and consists of resistances and capacitances arranged in a twin T design. The output of the amplifier is fed to the balanced detector consisting of a double triode 25 the anodes of the detector being connected to high tension positive through potentiometer P4. Finally measuring instruments, namely a D.C. voltmeter 26 and recording instrument 27 with relay 28 are arranged in parallel with one another. Alternatively as shown in Fig. 3 the sensitivity of the arrangement is increased by a phase-selecting device associated with the detector wherein the cathodes of the triodes 25 are not directly earthed as in Fig. 2 but are connected to the secondary winding of an audio-frequency transformer T4, the primary winding of which receives a reference voltage from the oscillator. The mains frequency heating current may be supplied through a transformer to the same diagonal of the bridge as the secondary winding of transformer 17 (Fig. 2) suitable capacitances being inserted to prevent propagation of the heating current towards the oscillator or of audio-frequency current towards the heating current transformer. A proportion of 4 per cent of oxygen can result in a deviation of 26 volts by employing a magnet having an induction of 4,000 gauss and an amplifier having a gain of 1,000 at 1,000 cycles per second or smaller concentrations down to 0.1 per cent may be measured by employing a stronger magnet for example 10,000 gauss.