We have seen that the current generated in the coil is alternating. Alternating current is very valuable for lighting and power, but there are cases in electroplating and charging storage batteries where it is absolutely necessary to have the current flow in the same direction. To do this, it is necessary to add to the dynamo a device called a commutator, the object of which is to make the current flow in one direction in the external circuit, regardless of the fact that the current reverses twice in every revolution.

A commutator consists of copper bars which are arranged in circular form and separated or insulated from each other by thin plates of mica. The bars connect with the armature wires, so that the current, as fast as it is generated, flows from the armature to the segments of the commutator.