The first half turn

Figure 13.14 shows a motor with all these parts put together. To understand how it works, figure 13.15 shows the parts in simple form: the coil is drawn as a single turn ABCD, with a magnetic pole on each side.

Figure to be added

Figure 13.14 — the main parts of a DC motor: the armature on its shaft, the split rings, the brushes pressing on them, and the magnetic poles in the case.

The coil is joined to the two split rings X and Y, and the two brushes P and Q are connected to the battery S.

Figure to be added

Figure 13.15 — a single loop ABCD between an N pole (left) and an S pole (right); its ends go to split rings X and Y; brush P (+) touches X and brush Q (−) touches Y; the direction of rotation is marked.

Following the current

When the current is switched on, it enters split ring X from brush P, travels along the loop in the direction ABCD, reaches split ring Y, and leaves through brush Q.
In the part of the loop in the magnetic field, the current flows in the sides AB and CD.
Use Fleming's left-hand rule on AB and CD to find the forces: the force on AB is downwards and the force on CD is upwards. The couple they make turns the armature clockwise.

Why the sides BC and DA do not count

The loop has four sides, but only AB and CD run across the field between the poles. BC runs along the direction of the field at the far end, and DA leads out to the rings; a wire lying along the field feels no force, and the wires to the rings are outside the gap. So the whole turning effect comes from AB and CD. Because the currents in them go in opposite directions — out along one side and back along the other — the forces on them are also opposite, one down and one up, which is exactly what makes a couple rather than a push in one direction.

Down on AB, up on CD: clockwise

With current ABCD, the left-hand rule pushes AB down and CD up, so the armature turns clockwise.