lf-starters" have a battery and motor to spin the coil for you
until the engine begins to go; then the engine turns the coil of the
magneto.

HOW OLD-FASHIONED TELEPHONES ARE RUNG. The old-fashioned telephones,
still often used in the country, have little cranks that you turn to
ring for central. The crank turns a coil of wire between the poles of
the magnet and generates the electricity for ringing the bell.
These little dynamos, like those in automobiles, are usually called
magnetos.

[Illustration: FIG. 118. The magneto in an automobile is a small
dynamo.]

ALTERNATING CURRENT. For the sake of simplicity and convenience we
speak of electricity as always flowing in through one wire and out
through the other. With batteries this is actually the case. It is
also the case where people have what is called _direct-current_ (d.
c.) electricity. But it is easier to raise and lower the voltage
(pressure) of the current if instead of being direct it is
_alternating_; that is, if for one instant the electricity flows in
through one wire and out through the other, the next instant flowing
the opposite way, then the first way again, and so on. This kind of
current is called _alternating current_ (a. c.), because the current
alternates, coming in the upper wire and out of the lower for a
fraction of a second; then coming in the lower and out of the upper
for the next fraction of a second; then coming in the upper again
and out of the lower for a fraction of a second; and so on, back
and forth, all the time. For heating and lighting, this alternating
current is just as good as the direct current, and it is probably what
you have in your own home. For charging storage batteries and making
electromagnets, separating water into two gases, and for running
certain kinds of motors, however, the direct current is necessary.
Find out whether the current in your laboratory is direct or
alternating.

_APPLICATION 49._ Explain why we need fuel or water to
generate large currents of electricity; how we can get small
amounts of electricity to flow without using dynamos; why
automobiles must be cranked unless they have batteries to
start them.

INFERENCE EXERCISE

Explain the following:

301. Mexican water jars are made of porous clay; the water
that seeps through keeps the water inside cool.

302. When you crank an automobile, electricity is generated.

303. Potatoes will not cook any more