current; and if, on the other hand, the current be sent through it
in the reverse direction to the arrow, it will move towards the
reader. This is the principle of the equally well-known electric
motor. Figure 39 shows a simple method of remembering these
directions.

Let the right hand rest on the north pole of a magnet and the
forefinger be extended in the direction of the lines of force,
then the outstretched thumb will indicate the direction in which
the wire or conductor moves and the bent middle finger the
direction of the current. These three digits, as will be noticed,
are all at right angles to each other, and this relation is the
best for inducing the strongest current in a dynamo or the most
energetic movement of the conductor in an electric motor.

Of course in a dynamo-electric generator the stronger the magnetic
field, the less the resistance of the conductor, and the faster it
is moved across the lines of force, that is to say, the more lines
it cuts in a second the stronger is the current produced.
Similarly in an electric motor, the stronger the current and
magnetic field the faster will the conductor move.

The most convenient motion to give the conductor in practice is
one of rotation, and hence the dynamo usually consists of a coil
or series of coils of insulated wire termed the "armature," which
is mounted on a spindle and rapidly rotated in a strong magnetic
field between the poles of powerful magnets. Currents are
generated in the coils, now in one direction then in another, as
they revolve or cross different parts of the field; and, by means
of a device termed a commutator, these currents can be collected