What causes a battery to produce electricity?
What makes electricity come into our houses?
The kind of electricity you get from rubbing (friction) is not of much
practical use, you remember. Men had to find a way to get a steady
current of electricity before they could make electricity do any work
for them. The difference between static electricity--when it leaps
from one thing to another--and flowing electricity is a good deal like
the difference between a short shower of rain and a river. Both rain
and river are water, and the water of each is moving from one place to
another; but you cannot get the raindrops to make any really practical
machine go, while the rivers can do real work by turning the wheels in
factories and mills.
Within the past century two devices for making electricity flow and
do work have been perfected: One of these is the electric battery; the
other is the dynamo.
THE ELECTRIC BATTERY. A battery consists of two pieces of different
kinds of metal, or a metal and some carbon, in a chemical solution.
If you hang a piece of zinc and a carbon, such as comes from an arc
light, in some water, and then dissolve sal ammoniac in the water, you
will have a battery. Some of the molecules of the sal ammoniac divide
into two parts when the sal ammoniac gets into the water, and the
molecules continue to divide as long as the battery is in use or
until it "wears out." One part of each molecule has an unusually large
number of electrons; the other part has unusually few. The parts with
unusually large numbers of electrons gather around the zinc; so the
zinc is _negatively charged_,--it has more than the ordinary number of
electrons. The part of the sal ammoniac with unusually few electrons
goes over to the carbon; so the carbon is _positively charged_,--it
has fewer than the ordinary number of electrons.
MAKING THE CURRENT FLOW. Now if we can make some kind of bridge
between the carbon and the zinc, the electrons will flow from the
place where there are many to the place where there are few. Electrons
can flow through copper wire very easily. So if we fasten one end
of the copper wire to the carbon and the other end to the zinc, the
electrons will flow from the zinc to the carbon as long as there are
more electrons on the zinc; that is, until the battery wears out.
Therefore we have a steady flow of electricity through the wire. While
the electricity is flowing from one pole to the other, we can make it
do work.
EXPERIMENT 64. Set up two or three Samson cells. They consist
of a glass jar, an open zinc cylinder, and a smaller carbon
cylinder. Dissolve a little over half a cup of sal ammoniac
in water and put it into the glass jar; then fill the jar with
water up to the line that is marked on it. Put the carbon and
zinc which are attached to the black jar cover into the jar.
Be careful not to let the carbon touch the zinc. One of these
cells will probably not be strong enough to ring a doorbell
for you; so connect two or three together in series as
follows:
Fasten a piece of copper wire from the carbon of the first
cell to the zinc of the second. If you have three cells,
fasten another piece of wire from the carbon of the second
cell to the zinc of the third, as shown in Figure 111.
Fasten one end of a copper wire to the zinc of the first
cell and the other end of this wire to one binding post of an
electric bell. Fasten one end of another piece of copper wire
to the carbon of the third cell, if you have three, and touch
the other end of this wire to the free binding post of the
electric bell. If you have everything connected rightly, the
bell should ring.
[Illustration: FIG. 111. A wet battery of three cells connected to ring
a bell.]
DIFFERENT KINDS OF BATTERIES. There are many different kinds of
batteries. The one you have just made is a simple one frequently used
for doorbells. Other batteries are more complicated. Some are made
with copper and zinc in a solution of copper sulfate; some, even, are
made by letting electricity from a dynamo run through a solution from
one lead plate to another until a chemical substance is stored on one
of them; then, when the two lead plates are connected by a wire, the
electrons run from one to the other. This kind of battery is called a
_storage battery_, and it is much used in submarines and automobiles.
[Illustration: FIG. 112. A battery of three dry cells.]
But all the different batteries work on the same general principle: A
chemical solution divides into two parts, one with many electrons and
the other with a less number. One part of the solution gathers on one
pole (piece of metal in the solution) and charges it positively; the
other part gathers on the other pole and charges it negatively. Then
the electricity flows from one pole to the other.
POSITIVE AND NEGATIVE POLES. Before people knew anything about
electrons, they knew that electricity flowed from one pole of a
battery to the other. But they always said that it flowed from the
carbon to the zinc; and they called the carbon the positive pole and
the zinc the negative. Although we now know that the electrons flow
from the zinc to the carbon, it is much more convenient to use the old
way of speaking, as was explained on page 199. Practically, it makes
no difference which way the electrons are going as long as a current
of electricity is flowing through the wire from one pole of the
battery to the other pole. So every one speaks of electricity as
flowing from the positive pole of a battery (usually the carbon
or copper) to the negative pole (usually the zinc), although the
electrons actually move in the other direction.
[Illustration: FIG. 113. A storage battery.]
Batteries make enough electricity flow to do a good deal of work. But
they are rather expensive, and it takes a great many to give a flow of
electricity sufficient for really heavy work, such as running street
cars or lighting a city. Fortunately there is another way of getting
large amounts of electricity to flow. This is by means of dynamos.
HOW A DYNAMO MAKES A CURRENT FLOW. To understand a dynamo, you must
first realize that there are countless electrons in the world--perhaps
all things are made entirely of them. But you remember that when we
want to get these electrons to do work we must make them flow. This
can be done by spinning a loop of wire between the poles of a magnet.
Whenever a loop of wire is turned between the two poles of a magnet,
the magnetism pushes the electrons that are already in the wire around
and around the loop. As long as we keep the loop spinning, a current
of electricity flows.
[Illustration: FIG. 114. Spinning loops of wire between the poles of a
magnet causes a current of electricity to flow through the wire.]
If only one loop of wire is spun between the poles of a magnet, the
current is very feeble. If you loop the wire around twice, as shown in
Figure 114, the magnet acts on twice as much of the wire at the same
time; so the current is stronger. If a very long piece of wire is used
and is looped around many times, and the whole coil is spun rapidly
between the poles of a powerful magnet, myriads of the electrons
in the wire rush around and around the loops--a powerful current of
electricity flows through the wire.
[Illustration: FIG. 115. The more loops there are, the stronger the
current.]
Now suppose you bring one loop of the long wire out, as shown in
Figure 115, and suppose you spin the rest of the loops between the
poles of the magnet. Then, to flow through the loops by the magnet the
electricity will have to go clear out through the long loop and back
again. While it is flowing through this long loop, we can make it
work. We can cut the long loop and attach one broken end to one part
of an electric lamp and the other end to the other part, so that the
electricity has to flow through the lamp in order to get back to the
spinning coil of wire, as shown in Figure 116. Such an arrangement as
this is really an extremely simple dynamo.
[Illustration: FIG. 116. If the electricity passes through a lamp on
its way around the circuit the filament of the lamp glows.]
[Illustration: FIG. 117. A dynamo in an electric light plant.]
You could make a dynamo that would actually work, by arranging such an
apparatus so that the coil would spin between the poles of the
magnet. But of course the big commercial dynamos are very much more
complicated in their construction. Figure 116 shows only the general
principle on which they work. The main point to note is that by
spinning a coil of wire between the poles of a magnet, you can make
electricity flow rapidly through the wire. And it is in this way that
most of the electricity we use is made.
The power spinning the coil of wire is sometimes steam, and sometimes
gasoline or distillate; and water power is very often used. A
large amount of our electricity comes from places where there are
waterfalls. Niagara, for instance, turns great dynamos and generates
an enormous amount of electricity.
WHY MANY AUTOMOBILES HAVE TO BE CRANKED. In an automobile, the magneto
is a little dynamo that makes the sparks which explode the gasoline.
While the automobile is going the engine spins the coil of wire
between the magnets, but at starting you have to spin the coil
yourself; and doing that is called "cranking" the automobile.
"Self-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 quickly in water that is
boiling violently than in water that is boiling gently.
304. When you brush your hair on a winter morning, it
sometimes stands up and flies apart more and more as you
continue to brush it.
305. You cannot see a person clearly through a ground-glass
window, although it lets most of the light through.
306. There is a layer of coarse, _light-colored_ gravel over
the tar on roofs, to keep the tar from melting.
307. It is very easy to slip on a well-waxed hardwood floor.
308. If you have a silver filling in one of your teeth and
you touch the filling with a fork or spoon, you get a slight
shock.
309. You can shake a thing down into a bottle when it will not
slip down by itself.
310. If you rub a needle across one pole of a magnet three or
four times in the same direction, then float it on a cork in
water one end of the needle will point north.