What is electricity?
What makes thunder and lightning?
Why does the barrel or cap of a fountain pen pick up small
bits of paper after it has been rubbed on your coat sleeve?
Why do sparks fly from the fur of a cat when you stroke it in
the dark?
The Greeks, 2000 years ago, knew that there was such a thing as
electricity, and they used to get it by rubbing amber with silk. In
the past century men have learned how to make electricity do all sorts
of useful work: making boats and cars and automobiles go, ringing
bells, furnishing light, and, in the telephone and telegraph, carrying
messages. But no one knew what electricity really was until, within
the last 25 years, scientists found out.
ATOMS AND ELECTRONS. When we talked about molecules, we said that they
were as much smaller than a germ as a germ is smaller than a mountain.
Well, a molecule is made up, probably, of some things that are much
smaller still,--so small that people thought that nothing could be
smaller. Those unthinkably tiny things are called _atoms_; you will
hear more about them when you come to the parts of this book that tell
about chemistry.
But if you took the smallest atom in the world and divided it into
1700 pieces, each one of these would be about the size of a piece of
electricity.
Electricity is made up of the tiniest things known to man--things so
small that nobody really can think of their smallness. These little
pieces of electricity are called _electrons_, and for all their
smallness, scientists have been able to find out a good deal about
them. They have managed to get one electron all by itself on a droplet
of oil and they have seen how it made the oil behave. Of course
they could not see the electron, but they could tell from various
experiments that they had just one. Scientists know how many trillions
of electrons flow through an incandescent electric lamp in a second
and how many quadrillions of them it would take to weigh as much as a
feather. They know what the electrons do when they move, how fast they
can move, and what substances let electrons move through them easily
and what substances hold them back; and they know perfectly well
how to set them in motion. How the scientists came to know all these
things you will learn in the study of physics; it is a long story.
But you can find out some things about electrons yourself. The first
experiment is a simple one such as the Greeks used to do with amber.
EXPERIMENT 63. Rub a hard rubber comb on a piece of woolen
cloth. The sleeve of a woolen coat or sweater will do. Rub the
comb quickly in the same direction several times. Now hold it
over some small bits of paper or sawdust. What does it do
to them? Hold it over some one's hair. The rest of this
experiment will work well only on cool, clear days. Rub the
comb again, a dozen or more times in quick succession. Now
touch it gently to the lobe of your ear. Do you hear the snap
as the small spark jumps from the comb to your ear?
Pull a dry hair out of your head and hold it by one end.
Charge your comb by rubbing it again, and bring it near the
loose end of the hair. If the end of the hair clings to the
comb at first, leave it clinging until it flies off. Now try
to touch the hair with the comb. Next, pinch the end of the
hair between your thumb and finger and again bring the
charged comb near it. Is the hair attracted or repelled? After
touching the comb what does it do?
You can get the same effects by rubbing glass or amber on
silk.
[Illustration: FIG. 109. When the comb is rubbed on the coat, it becomes
charged with electricity.]
OBJECTS NEGATIVELY AND POSITIVELY CHARGED WITH ELECTRICITY. There are
probably electrons in everything. But when there is just the usual
number of electrons in an object, it acts in an ordinary way and we
say that it is not charged with electricity. If there are more than
the usual number of electrons on an object, however, we say that it is
_negatively charged_, or that it has a negative charge of electricity
on it. But if there are fewer electrons than usual in an object, we
say that it has a positive charge of electricity on it, or that it is
_positively charged_.
You might expect a "negative charge" to indicate fewer electrons than
usual, not more. But people called the charge "negative" long before
they knew anything about electrons; and it is easier to keep the
old name than to change all the books that have been written about
electricity. So we still call a charge "negative" when there are
unusually _many_ electrons, and we call it "positive" when there are
unusually _few_. A _negative charge_ means that more electrons are
present than usual. A _positive charge_ means that fewer electrons are
present than usual.
[Illustration: FIG. 110. The charged comb picks up pieces of paper.]
Before you rubbed your comb on wool, neither the comb nor the wool
was charged; both had just the usual number of electrons. But when you
rubbed them together, you rubbed some of the electrons off the wool on
to the comb. Then the comb had a negative charge; that is, it had too
many electrons--too many little particles of electricity.
When you brought the comb near the hair, the hair had fewer electrons
than the comb. Whenever one object has more electrons on it than
another, the two objects are pulled toward each other; so there was
an attraction between the comb and the hair, and the hair came over to
the comb. As soon as it touched the comb, some of the extra electrons
jumped from the comb to the hair. The electrons could not get off the
hair easily, so they stayed there. Electrons repel each other--drive
each other away. So when you had a number of electrons on the end of
the comb and a number on the end of the hair, they pushed each other
away, and the hair flew from the comb. But when you pinched the
hair, the electrons could get off it to your moist hand, which
lets electricity through it fairly easily. Then the comb had extra
electrons on it and the hair did not; so the comb pulled the hair over
toward it again.
When you brought the charged comb near your ear, some of the electrons
on the comb pushed the others off to your ear, and you heard them snap
as they rushed through the air, making it vibrate.
HOW LIGHTNING AND THUNDER ARE CAUSED. In thunderstorms the strong
currents of rising air blow some of the forming raindrops in the
clouds into bits of spray. The tinier droplets get more than their
share of electrons when this happens and are carried on up to higher
clouds. In this way clouds become charged with electricity. One
cloud has on it many more electrons than another cloud that is made,
perhaps, of lower, larger droplets. The electricity leaps from the
cloud that has the greater number of electrons to the cloud that has
the less number, or it leaps from the heavily charged cloud down to a
tree or house or the ground. You see the electricity leap and call it
_lightning_. Much more leaps, however, than leaped from the comb to
your ear, and so it makes a very much louder snap. The snap is caused
in this way: As the electric spark leaps through the air, it leaves
an empty space or vacuum immediately behind it. The air from all sides
rushes into the vacuum and collides there; then it bounces back. This
again leaves a partial vacuum; so the air rushes in once more, coming
from all sides at once, and again bounces back. This starts the air
vibrations which we call _sound_. Then the sound is echoed from cloud
to cloud and from the clouds to the earth and back again, and we call
it _thunder_.
The electricity you have been reading about and experimenting with in
this section is called _static electricity_. "Static" means standing
still. The electricity you rubbed up to the surface of the comb or
glass stayed still until it jumped to the bit of paper or hair; then
it stayed still on that. This was the only kind of electricity most
people knew anything about until the nineteenth century; and it is
not of any great use. Electricity must be flowing through things to do
work. That is why people could not invent electric cars and electric
lights and telephones before they knew how to make electricity flow
steadily rather than just to stand still on one thing until it jumped
across to another and stood there. In the next chapter we shall take
up the ways in which electrons are made to flow and to do work.
_APPLICATION 48._ Explain why the stroking of a cat's back
will sometimes cause sparks and make the cat's hairs stand
apart; why combing sometimes makes your hairs fly apart. Both
of these effects are best secured on a dry day, because on a
damp day the water particles in the air will let the electrons
pass to them as fast as they are rubbed up to the surface of
the hair.
INFERENCE EXERCISE
Explain the following:
291. If you shuffle your feet on a carpet in clear, cold
weather and then touch a person's nose or ear, a slight spark
passes from your finger and stings him.
292. If you stay out in the cold long, you get chilled
through.
293. The air and earth in a greenhouse are warmed by the sun
through the glass even when it is cold outside and when the
glass itself remains cold.
294. When you hold a blade of grass taut between your thumbs
and blow on it, you get a noise.
295. Shadows are usually black.
296. Some women keep magnets with which to find lost needles.
297. You can grasp objects much more firmly with pliers than
with your fingers.
298. If the glass in a mirror is uneven, the image of your
face is unnatural.
299. A sweater clings close to your body.
300. Kitchens, bathrooms, and hospitals should have painted
walls.
CHAPTER EIGHT
ELECTRICITY