7. 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

SECTION 33. _Making electricity flow._

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 copp