An electromechanical integrator is described which is an a.c. version, operating with 50 c/s signals, of the well-known velodyne principle. The difficulties peculiar to the a.c. velodyne are summarized and the features of the design, by which these are overcome, are indicated. The accuracy of integration is discussed, with special reference to the errors caused by Coulomb friction, to transient response and to temperature effects. Integrators of the type described have given satisfactory service for some years in a large special-purpose analogue computer.