clei and the planetary
electrons of each atom.

I hope this clears up the questions in your mind for I want to get along
to the vacuum tube. By a vacuum we mean a space which has very few atoms
or molecules in it, just as few as we can possibly get, with the best
methods of pumping and exhausting. For the present let's suppose that we
can get all the gas molecules, that is, all the air, out of a little
glass bulb.

The audion is a glass bulb like an electric light bulb which has in it a
thread, or filament, of metal. The ends of this filament extend out
through the glass so that we may connect a battery to them and pass a
current of electricity through the wire. If we do so the wire gets hot.

What do we mean when we say "the wire gets hot?" We mean that it feels
hot. It heats the glass bulb and we can feel it. But what do we mean in
words of electrons and atoms? To answer this we must start back a little
way.

In every bit of matter in our world the atoms and molecules are in very
rapid motion. In gases they can move anywhere; and do. That's why odors
travel so fast. In liquids most of the molecules or atoms have to do
their moving without getting out of the dish or above the surface. Not
all of them stay in, however, for some are always getting away from the
liquid and going out into the air above. That is why a dish of water
will dry up so quickly. The faster the molecules are going the better
chance they have of jumping clear away from the water like fish jumping
in the lake at sundown. Heating the liquid makes its molecules move
faster and so more of them are able to jump clear of the rest of the
liquid. That's why when we come in wet we hang our clothes where they
will get warm. The water in them evaporates more quickly when it is
heated because all we mean by "heating" is speeding up the molecules.

In a solid body the molecules can't get very far away from where they
start but they keep moving back and forth and around and around. The
hotter the body is, the faster are its molecules moving. Generally they
move a little farther when the body is hot than when it is cold. That
means they must have a little more room and that is why a body is larger
when hot than when cold. It expands with heating because its molecules
are moving more rapidly and slightly farther.

When a wire is heated its molecules and atoms are hurried up and they
dash back and forth faster than before. Now you know that a wire, like
the f