DC Generator:

A simple DC generator consists of an armature coil within a single turn of wire.  An armature coil cuts across the magnetic field to generate a voltage output.  As long as an overall path is present, current will flow by the circuit in the direction display through the arrows in Figure.  Within this coil position, commutator segment 1 contacts with brush 1, although commutator segment 2 is in contact along with brush 2.

Rotating the armature one-half turn in the clockwise direction causes the contacts among the commutator segments to be reversed. Now segment 1 is contacted through brush 2, and segment 2 is in contact along with brush 1.

Figure: Basic DC Generator

Due to this commutator action which side of the armature coil that is in contact along with either of the brushes is always cutting the magnetic field in the similar direction.  Brushes 1 and 2 have a constant pulsating and polarity DC is delivered to the load circuit.