Part 3 (1/2)

Sometimes we speak of the vacuu 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 oraph 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 filalass bulb these electrons which get free froo 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 nu You realize that this is what is happening inside an ordinary electric light bulb when its fila 4]

An ordinary electric light bulb, however, is not an audion although it is like one in the emission of electrons froht as I aiting for a train I picked up one of the Radio Supple There was a column of enquiries One letter told how its writer had tried to use an ordinary electric light bulb to receive radio signals

He had plenty of electrons in it but no way to control them and make their motions useful In an audion besides the filas One is a little sheet or plate of lass walls and the other is a little wire screen shaped like a gridiron and so called a ”grid” It also has a connecting wire leading through the glass Fig 4 shows an audion It will be5

There you see the filaht out frorid, _G_

[Illustration: Fig 5]

These three parts of the tube are sometimes called ”elements” Usually, however, they are called ”electrodes” and that is why the audion is spoken of as the ”three-electrode vacuum tube” An electrode is e call any piece of et at electrons (or ions) to control their motions Let us see how it does so

To start with, we shall forget the grid and think of a tube with only a filament and a plate in it--a two-electrode tube We shall represent it as in Fig 6 and show the battery which heats the fila a battery each cell is represented by a short heavy line and a longer lighter line The heavy line stands for the negative plate and the longer line for the positive plate We shall call the battery which heats the filament the ”filament battery” or sometimes the ”A-battery” As you see, it is formed by several battery cells connected in series

[Illustration: Fig 6]

Soether and why For the present all you need to remember is that two batteries are in series if the positive plate of one is connected to the negative plate of the other If the batteries are alike they will pull an electron just twice as hard as either could alone

[Illustration: Pl IV--Radiotron (Courtesy of Radio Corporation of America)]

To heat the filament of an audion, such as you will probably use in your set, will require three storage-battery cells, like the one I described in enerally use storage batteries of about the same size as those in the automobile If you will look at the automobile battery you will see that it is made of three cells connected in series That battery would do very well for the filament circuit

By the way, do you knohat a ”circuit” is? The word comes from the same Latin word as our word ”circus” The Ro at their circuses and built race tracks around which the chariots could go A circuit, therefore, is a path or track around which so can race; and an electrical circuit is a path around which electrons can race The fila 6 forine another battery formed by several cells in series which we shall connect to the tube as in Fig 7 All the positive and negative terminals of these batteries are connected in pairs, the positive of one cell to the negative of the next, except for one positive and one negative The re positive terminal is the positive ter by this series connection We then connect this positive terative terure This new battery we shall call the ”plate battery” or the ”B-battery”

Nohat's going to happen? The B-battery ant to take in electrons at its positive terative terminal

The positive is connected to the plate in the vacuuure and so draws some of the electrons of the plate away fro to the ato in the space between the ato when the battery called them That leaves the plate with less than its proper number of electrons; that is, leaves it positive So the plate iming about in the vacuu in the tube? The filah there were already in the tube so many electrons that just as many crowded back into the filament each second as the fila electrons, although it was busy sending theain

When the B-battery gets to work all this is changed The B-battery attracts electrons to the plate and so reduces the crowd in the tube

Then there are not asback into the filament as there were before and so the filaets back

Suppose that, before the B-battery was connected to the plate, each tiny length of the fila 1000 back each second There was no net change Now, suppose that the B-battery takes away 100 of these each second Then only 900 get back to the filament and there is a net loss froth of filament sends into the vacuum 100 electrons which are taken out at the plate From each little bit of filament there is a stream of electrons to the plate Millions of electrons, therefore, stream across from filament to plate That is, there is a current of electricity between fila as the A-battery and the B-battery do their work