The 'Metadyne' and the 'Amplidyne' are special-purpose dc generators historically used as high power electro-mechanical amplifiers in control systems. In use, such machines are driven at constant speed by a motor. The electrical output is varied by control of field excitation, as in a Ward-Leonard system. The Metadyne and the Amplidyne include an arrangement of cross-connected brushes on one axis and a further set of brushes on a perpendicular axis. This arrangement allows the machine to provide very high gain, that is, large changes of output may be controlled by small changes in the controlling field current

The 'Rototrol' is a two-stage machine with static and dynamic characteristics similar to those of the 'Amplidyne'. The 'Rototrol' may also be operated as a three-stage machine (also known as a 'Magnicon') in which the output is further used to excite a pole winding.