Although generators made earlier in the 19th century had produced alternating current, (AC) those constructed later concentrated on the generation of direct current (DC) which at that time was considered more useful. From 1873 it had been known that the current generated by a dynamo, passed through a similar machine in a reverse direction, would rotate the armature of that machine. This principle gave rise to the electromotor. Well made dynamos always duplicate as efficient electromotors, although the reverse does not hold true. Edison's motor of 1879 featured long horizontal field electromagnets (1), with attached pole pieces (2), between which the armature revolved (3) on its shaft (4). The brushes (5) ran against a split ring commutator (6) thereby collecting the DC current and passing it to the terminals (7). A drive pulley (8) was located on the motor shaft, in order to transport the motion produced.
A motor is a mechanism that converts energy (such as heat or electricity) into useful work. The term is sometimes applied to the internal combustion engine (which converts heat produced by burning gases into reciprocating or rotary motion), but is more often applied to the electric motor (which converts electrical energy into rotary motion).
Rockets are motors that can leave the Earth's atmosphere because they carry both fuel and oxidizer. Other types of motor presently being developed include the linear motor, a new motor proposed for high-speed train propulsion, and ion engines, which employ ion propulsion to drive the spacecraft.