ilament of a lamp, gets hot when the "electricity is turned on,"
that is, when there is a stream of electrons passing through it. Why
does it get hot? Because when the electrons stream through it they bump
and jostle their way along like rude boys on a crowded sidewalk. The
atoms have to step a bit more lively to keep out of the way. These more
rapid motions of the atoms we recognize by the wire growing hotter.

That is why an electric current heats a wire through which it is
flowing. Now what happens to the electrons, the rude boys who are
dodging their way along the sidewalk? Some of them are going so fast and
so carelessly that they will have to dodge out into the gutter and off
the sidewalk entirely. The more boys that are rushing along and the
faster they are going the more of them will be turned aside and plunge
off the sidewalks.

The greater and faster the stream of electrons, that is the more current
which is flowing through the wire, the more electrons will be "emitted,"
that is, thrown out of the wire. If you could watch them you would see
them shooting out of the wire, here, there, and all along its length,
and going in every direction. The number which shoot out each second
isn't very large until they have stirred things up so that the wire is
just about red hot.

What becomes of them? Sometimes they don't get very far away from the
wire and so come back inside again. They scoot off the sidewalk and on
again just as boys do in dodging their way along. Some of them start
away as if they were going for good.

If the wire is in a vacuum tube, as it is in the case of the audion,
they can't get very far away. Of course there is lots of room; but they
are going so fast that they need more room just as older boys who run
fast need a larger play ground than do the little tots. By and by there
gets to be so many of them outside that they have to dodge each other
and some of them are always dodging back into the wire while new
electrons are shooting out from it.

When there are just as many electrons dodging back into the wire each
second as are being emitted from it the vacuum in the tube has all the
electrons it can hold. We might say it is "saturated" with electrons,
which means, in slang, "full up." If any more electrons are to get out
of the filament just as many others which are already outside have to go
back inside. Or else they have got to be taken away somewhere else.

What I have just told you about