The paper describes the development of a new type of a.c. generator in which the moving member travels between a d.c. pole structure in a straight line with reciprocating motion. Tha induced e.m.f. appears in the moving member, which consists of a single loop of conducting material which embraces and moves along the core. The core carries stationary coils which experience an induced alternating e.m.f. by transformer action from the moving loop. These coils are not associated with the air-gap between the d.c. poles and are more easily cooled than the windings of a rotary machine. The loop, which is confined to the air-gap, can be run at high temperature, since it carries no insulation. The machine can be used as a generator to convert mechanical power supplied directly from a piston. It can also be used as a synchronous motor for such purposes as the driving of compressors without cranks; when used in this manner the machine is self-starting. Essentially a single-phase machine, the equivalent of multi-polar rotary machines can be constructed which will generate e.m.f.'s at 50c/swith mechanical oscillations at frequencies lower than 3 000 per minute.The construction of the machine is simple, since the core is made up entirely from rectangular stampings, while the windings consist of four transformer-type coils and a conducting loop.An experimental machine is described, development of which consisted largely of testing devices to minimize the internal impedance Test results are given.The paper includes a proposed design for a 33 kVA generator.