The instrument operates as a force balance system in which the torque caused by the flow of air past a four-bladed vane assembly is opposed by an equal torque produced by the flow of current in a coil attached to the vane and suspended in a permanent magnetic field. These opposing forces remain in equilibrium until a variation in air flow takes place, when the vane assembly (which, mechanically, is not restrained) begins to turn. A mirror attached to this assembly, and which rotates with it, causes the image of an illuminated slit to move across a photocell which, in turn, controls the amount of current flowing in the coil so as to oppose its rotation until the two torques are again equal. This current is proportional to the square of the air flow and is used to provide the calibrated output, for application to the recorder.The performance of the prototype instrument built in the British Coal Utilisation Research Association laboratories is as follows:(i) Range: there are three ranges, selected by a switch: 10-100 ft³ h^-1, 50-500 ft³ h^-1, and 100-1000 ft³ h^-1. (ii) Stability: because of the force balance principle used, the instrument is largely independent of mains and lamp voltage changes; stability checks over a period of 100 hours gave variations of ±1% in calibration. (iii) Output: the coil current provides an output which may be recorded directly on a recording ammeter. Alternatively, this current may be used to develop a voltage across a resistor and so provide an output suitable for application to a standard potentiometric recorder.