Measuring the diameter of a fine textile filament by its electrical capacitance effect between the fixed plates of a capacitor presents the twin problems of the very small capacitance change to be measured and the high degree of stability to be maintained in the capacitor.The present instrument is insensitive to changes in the standing capacitance and achieves this by vibrating the yarn in and out of the gap. The capacitance between the two electrodes is therefore varying at the frequency of the vibrating yarn, and a variable capacitor is switched into circuit in phase with the yarn and used to balance this change in capacitance in a bridge circuit. The output from the bridge is amplified and used to drive a motor which balances the bridge by adjusting the variable capacitor. The movement of the motor also drives a pen recorder. The bridge circuit conveniently provides several measuring ranges and allows the measuring head to be one of several connected to the main instrument by long cables.Synthetic-fibre manufacture involves numerous identical production units in parallel. Monitoring and control gear is therefore fed from a multiplicity of transducers which must be matched, reliable and capable of simple routine test and replacement without upsetting their related circuits.