The paper traces the development of inherent control systems in special electrical machinery over a period of more than 40 years. Starting from the concept of the 3-coil dynamo, the measures taken to make a successful constant-current d.c. generator are later shown to be applicable to constant-voltage constant-current d.c. convertors and to the control of alternator voltage, the voltage of variable-speed d.c. machines, speed, and similar applications.The development of these convertors led, in turn, to amplifying dynamos. Two basic types of convertor, and the amplifying dynamos developed from them, are briefly compared. One of them, namely the Magnicon, is treated in greater detail because of the authors' experience of this machine and because it is not very well documented in technical articles.A mathematical example of the oscillation phenomena occurring with these machines is given in the Appendices to the paper.Examples of the uses of the various machines developed cover constant-current auxiliary systems for ships, constant-voltage alternators, d. c. variable-speed machines and constant-speed control.