While at Graz Technical University, Tesla had seen a
demonstration of a generator run as a direct-current (DC) motor. Direct current is electric current that
flows in only one direction. During the
demonstration, the brushes and the commutator of the motor sparked
violently. The brushes are devices that
conduct the current in a DC motor. The
commutator continually reverses the current so that the motor continues to
rotate in one direction. Tesla believed
a motor without a commutator could be devised.
In 1881, while walking in a park, Tesla suddenly got an idea for a
simple way to produce such a device. In
1883, while on assignment for Continental Edison in Strasbourg, France, Tesla
used his spare time to build his first polyphase (out-of-step) AC motor. In such a motor, coils are arranged so that
when out-of-step alternating currents energize them, the resulting magnetic
field rotates at a predetermined speed.