If a current-carrying wire is placed in a magnetic field as in Figure 18, notice what happens to the
lines of force that are moving from the north pole to the south pole of the magnet. They are
forced to bend, just as the stretched rubberbands were forced to bend when you pressed on them
with your finger. The lines of force traveling from north to south bend down in this case because
they are pushed downward by the counterclockwise rotation of the lines of force around the
current-carrying wire. Because the lines of force from the north to south pole pieces of the magnet
try to straighten out like the rubberbands, they force the current-carrying wire up (note the arrow).
The current is moving in the opposite direction in the wire, and the magnet's lines of force push
down on this wire.
In the starter motor, like the generator, increasing the strength of the pole shoes will increase the
number of lines of force. Likewise, increasing the current flow through the wire will increase the
strength of the magnetic field around the wire.
When these magnetic forces oppose each other, they try to push each other away. The opposing
forces can be very great if the wire is carrying enough current to make the magnetic field very
Now let us bend a wire to form a loop and place the loop in a magnetic field. Nothing happens
until we send current through the loop. If we send current flowing through the loop, the magnet's
lines of force push up on the right side of the loop and down on the left side. This produces the
torque to rotate the entire loop counterclockwise (to the left). Actually, the loop would probably
move only one-fourth of a revolution (90) because it would be out of the magnetic field of the
magnet. The loop would then be straight up and down instead of straight across as shown.
To get continuous rotation, we need a magnetic field large enough to contain the loop. We would
also need commutator bars and brushes like we had in the generator.
Of course, a single loop would not produce enough torque to crank the engine. But, by using many
loops, each with its own commutator bars, we can have a cranking motor that will produce all of
the torque needed.