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
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
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.
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