A transmission dynamometer is described which was designed for use with belt-driven textile machinery. It is similar to the Ayrton & Perry instrument, and consists of a pulley wheel mounted on a bush which is fixed rigidly to the driving shaft in place of the ordinary fast pulley. The pulley wheel and bush are connected together by springs and their relative motion is observed by a stroboscopic method. The method of calibration is described and typical working results given.