t "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