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 i