English: The Ayrton and Perry transmission dynamometer or spring coupling. The relative angular displacement is proportional to the radius of the circle described by the end of a light lever operated by mechanism between the spring-connected parts. By a device used by W. E. Dalby the change in form of the spring is shown on a fixed indicator, which may be placed in any convenient position. Two equal sprocket wheels Q₁, Q₂, are fastened, the one to the spring pulley, the other to the shaft. An endless band is placed over them to form two loops, which during rotation remain at the same distance apart, unless relative angular displacement occurs between Q₁ and Q₂ due to a change in form of the spring. The change in the distance d is proportional to the change in the torque transmitted from the shaft to the pulley.