Motors
A current carrying wire experiences a force in a magnetic field. If the wire is in the form of a coil the opposite sides of the coil will experience forces in opposite directions. The coil will experience a torque.

For the coil to rotate however we will need to reverse the direction of the forces every half cycle so that the sense of the torque stays the same.

There are two ways of doing this:

Use an a.c. supply.
The current is supplied using slip rings and carbon brushes. The coil will rotate at the same frequency as the a.c. supply.

Use a commutator
This is a device which reverses the current every half cycle. The diagram below shows a split ring commutator. The more current we put in the coil the faster it will go.