commutator

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A cylindrical arrangement of insulated metal bars connected to the coils of a direct-current electric motor or generator, providing a unidirectional current from the generator or a reversal of current into the coils of the motor.
2. n. Mathematics In a commutative or noncommutative group, an element of the form ghg^-1h^-1 where g and h are elements of the group. If g and h commute, the commutator is the identity element.

Century Dictionary and Cyclopedia

1. n. An apparatus used in connection with many electrical instruments for reversing the currents from the battery without changing the arrangement of the conductors from the poles: as, Bertin's commutator. In the magneto-electric or dynamo-electric machines (see electric), a commutator is ordinarily employed to regulate the direction of the current through the external circuit.
2. n. A contrivance for varying the strength of an electric current by bringing either a portion or the whole of the voltaic cells in a battery into the circuit.
3. n. In electricity, that part of a continuous-current dynamo-electric machine which, by revolving in contact with the brushes, turns or directs the electrical actions in the armature coils of the machine so as to make the current and the voltage in the external circuit continuous. It usually consists of a large number of copper bars, connected with the armature coils of the machine, and separated from each other by mica insulation.
4. n. In photography, a device for automatically effecting the exposure of a number of photographic plates at various predetermined instants during a total eclipse of the sun.

Wiktionary

1. n. an electrical switch, in a generator or motor, that periodically reverses the direction of an electric current
2. n. mathematics (of a group) an element of the form ghg−1h−1 where g and h are elements of the group; it is equal to the group's identity if and only if g and h commute
3. n. mathematics (of a ring) an element of the form ab-ba, where a and b are elements of the ring, it is identical to the ring's zero element if and only if a and b commute

GNU Webster's 1913

1. n. (Elec.) A piece of apparatus used for reversing the direction of an electrical current; an attachment to certain electrical machines, by means of which alternating currents are made to be continuous or to have the same direction. It may be attached to the end of the spindle of an electric motor, where a brush is in contact sequentially with the parts of the spindle that conduct current to the different windings of the motor.

WordNet 3.0

1. n. switch for reversing the direction of an electric current