S. pole faces the N. of the magnet, and _vice versa_. The
lines of force of the coil and the magnet are now parallel. But the
momentum of revolution carries the coil on, and suddenly the commutator
reverses its polarity, and a further half-revolution takes place. Then
comes a further reversal, and so on _ad infinitum_. The rotation of the
motor is therefore merely a question of repulsion and attraction of like
and unlike poles. An ordinary compass needle may be converted into a
tiny motor by presenting the N. and S. poles of a magnet to its S. and
N. poles alternately every half-revolution.

In construction and winding a motor is practically the same as a dynamo.
In fact, either machine can perform either function, though perhaps not
equally well adapted for both. Motors may be run with direct or
alternating current, according to their construction.

On electric cars the motor is generally suspended from the wheel truck,
and a small pinion on the armature shaft gears with a large pinion on a
wheel axle. One great advantage of electric traction is that every
vehicle of a train can carry its own motor, so that the whole weight of
the train may be used to get a grip on the rails when starting. Where a
single steam locomotive is used, the adhesion of its driving-wheels only
is available for overcoming the inertia of the load; and the whole
strain of starting is thrown on to the foremost couplings. Other
advantages may be summed up as follows:--(1) Ease of starting and rapid
acceleration; (2) absence of waste of energy (in the shape of burning
fuel) when the vehicles are at rest; (3) absence of smoke and smell.

ELECTRIC LIGHTING.

Dynamos are used to generate current for two main purposes--(1) To
supply power to motors of all kinds; (2) to light our houses, factories,
and streets. In private houses and theatres incandescent lamps are
generally used; in the open air, in shops, and in larger buildings, such
as railway stations, the arc lamp is more often found.

INCANDESCENT LAMP.

If you take a piece of very fine iron wire and lay it across the
terminals of an accumulator, it becomes white hot and melts, owing to
the heat generated by its resistance to the current. A piece of fine
platinum wire would become white hot without melting, and would give out
an intense light. Here we have the principle of the glow or incandescent
lamp--namely, the interposition in an electric circuit of a conductor
which at once offers a