Unlike the constant potential of a battery, a DC generator creates a series of current fluctuations. This is known as pulsating direct current. For each half rotation of the armature, the current starts at zero with the wire moving parallel to the lines of flux. The current increases to its maximum level at the point where the wire cuts through the lines of flux at a 90° angle. The current then returns to zero a quarter of a rotation later when the wire again moves parallel to the lines of flux.
At this point, the armature has completed one half rotation through the magnetic field, starting at zero current, moving to its maximum current and then returning to zero. Now, however, things get a little tricky as the rotor begins its second half of a rotation.
The wire loop, through which current passed in one direction during the first half rotation, will now be "upside down" relative to the magnetic field for the next half of rotation. So the forces that induced current to flow through it in one direction during the first half rotation will now be pushing current in the opposite direction for the second half rotation.
If this sounds confusing, hold out your hand and picture a magnetic field inducing current to flow from your thumb to your small finger, just as it would in the wire loop. Now rotate your wrist half a turn and you'll see that the same magnetic field will now induce current to flow through your hand in the opposite direction, from your small finger to your thumb.
Because of this, DC generators require special split-ring commutators to draw current from the end of the wire loop as the armature rotates. These connections pass current to a different contact (or brush) for each half of the armature cycle. That is, each brush makes contact with each end of the wire for only half a rotation. So current reaching each brush is always flowing in the same direction, regardless of the position of the armature.
