In a generator the commutator picks off the current generated in the windings, reversing the direction of the current with each half turn, serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit.

How does a commutator change polarity?

So the commutator in contact with the armature also rotates. The wire on the left side has the current direction same. So the armature keeps on rotating in the same direction. Once the coil completes half a turn (just ignore we have a commutator), then the forces on the coil will be opposite.

What is used to reverse the direction of current in an electric motor?

The device which is used to reverse the direction of current in the coil of a motor is known as a commutator (or split ring). A device called a split ring or commutator consists of two conductive arcs separated by an insulator in an electric motor.