ansformation is
seen in Fig. 79. Alternating current of 5,000 volts pressure is produced
in the generating station and sent through conductors to a distant
station, where a transformer, B, reduces the pressure to 500 volts to
drive an alternating motor, C, which in turn operates a direct current
dynamo, D. This dynamo has its + terminal connected with the insulated
or "live" rail of an electric railway, and its - terminal with the wheel
rails, which are metallically united at the joints to act as a
"return." On its way from the live rail to the return the current passes
through the motors. In the case of trams the conductor is either a cable
carried overhead on standards, from which it passes to the motor through
a trolley arm, or a rail laid underground in a conduit between the
rails. In the top of the conduit is a slit through which an arm carrying
a contact shoe on the end projects from the car. The shoe rubs
continuously on the live rail as the car moves.

To return for a moment to the question of transformation of current.
"Why," it may be asked, "should we not send low-pressure _direct_
current to a distant station straight from the dynamo, instead of
altering its nature and pressure? Or, at any rate, why not use
high-pressure direct current, and transform _that_?" The answer is, that
to transmit a large amount of electrical energy at low pressure (or
voltage) would necessitate large volume (or _amperage_) and a big and
expensive copper conductor to carry it. High-pressure direct current is
not easily generated, since the sparking at the collecting brushes as
they pass over the commutator segments gives trouble. So engineers
prefer high-pressure alternating current, which is easily produced, and
can be sent through a small and inexpensive conductor with little loss.
Also its voltage can be transformed by apparatus having no revolving
parts.

THE ELECTRIC MOTOR.

Anybody who understands the dynamo will also be able to understand the
electric motor, which is merely a reversed dynamo.

Imagine in Fig. 70 a dynamo taking the place of the lamp and passing
current through the brushes and commutator into the coil _w x y z_. Now,
any coil through which current passes becomes a magnet with N. and S.
poles at either end. (In Fig. 70 we will assume that the N. pole is
below and the S. pole above the coil.) The coil poles therefore try to
seek the contrary poles of the permanent magnet, and the coil revolves
until its