electrons getting away from a heated
wire is very much like what happens when a liquid is heated. The
molecules of the liquid get away from the surface. If we cover a dish of
liquid which is being heated the liquid molecules can't get far away and
very soon the space between the surface of the liquid and the cover gets
saturated with them. Then every time another molecule escapes from the
surface of the liquid there must be some molecule which goes back into
the liquid. There is then just as much condensation back into liquid as
there is evaporation from it. That's why in cooking they put covers over
the vessels when they don't want the liquid all to "boil away."

Sometimes we speak of the vacuum tube in the same words we would use in
describing evaporation of a liquid. The molecules of the liquid which
have escaped form what is called a "vapor" of the liquid. As you know
there is usually considerable water vapor in the air. We say then that
electrons are "boiled out" of the filament and that there is a "vapor of
electrons" in the tube.

That is enough for this letter. Next time I shall tell you how use is
made of these electrons which have been boiled out and are free in the
space around the filament.

[Footnote 2: If the reader has omitted Letters 3 and 4 he should omit
this paragraph and the next.]

LETTER 6

THE AUDION

DEAR SON:

In my last letter I told how electrons are boiled out of a heated
filament. The hotter the filament the more electrons are emitted each
second. If the temperature is kept steady, or constant as we say, then
there are emitted each second just the same number of electrons. When
the filament is enclosed in a vessel or glass bulb these electrons which
get free from it cannot go very far away. Some of them, therefore, have
to come back to the filament and the number which returns each second is
just equal to the number which is leaving. You realize that this is what
is happening inside an ordinary electric light bulb when its filament is
being heated.

[Illustration: Fig 4]

An ordinary electric light bulb, however, is not an audion although it
is like one in the emission of electrons from its filament. That reminds
me that last night as I was waiting for a train I picked up one of the
Radio Supplements which so many newspapers are now running. There was a
column of enquiries. One letter told how its writer had tried to use an
ordinary electric light bulb to receive radio signal