Letters of a Radio-Engineer to His Son Part 3

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 s.p.a.ce 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 gla.s.s 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.

[Ill.u.s.tration: 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 signals.

He had plenty of electrons in it but no way to control them and make their motions useful. In an audion besides the filament there are two other things. One is a little sheet or plate of metal with a connecting wire leading out through the gla.s.s 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 gla.s.s. Fig. 4 shows an audion. It will be most convenient, however, to represent an audion as in Fig. 5.

There you see the filament, _F_, with its two terminals brought out from the tube, the plate, _P_, and between these the grid, _G_.

[Ill.u.s.tration: 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 what we call any piece of metal or wire which is so placed as to let us get 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 filament by some lines as at _A_. In this way of representing 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.

[Ill.u.s.tration: Fig 6]

Sometime later I may tell you how to connect battery cells together 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.

[Ill.u.s.tration: 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 my fourth letter, all connected in series. We generally 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 know what a "circuit" is? The word comes from the same Latin word as our word "circus." The Romans were very fond of chariot racing at their circuses and built race tracks around which the chariots could go. A circuit, therefore, is a path or track around which something can race; and an electrical circuit is a path around which electrons can race. The filament, the A-battery and the connecting wires of Fig. 6 form a circuit.

[Ill.u.s.tration: Fig 7]

Let us imagine 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 remaining positive terminal is the positive terminal of the battery which we are making by this series connection. We then connect this positive terminal to the plate and the negative terminal to the filament as shown in the figure. This new battery we shall call the "plate battery" or the "B-battery."

Now what's going to happen? The B-battery will want to take in electrons at its positive terminal and to send them out at its negative terminal.

The positive is connected to the plate in the vacuum tube of the figure and so draws some of the electrons of the plate away from it. Where do these electrons come from? They used to belong to the atoms of the plate but they were out playing in the s.p.a.ce between the atoms, so that they came right along 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 immediately draws to itself some of the electrons which are dodging about in the vacuum around it.

Do you remember what was happening in the tube? The filament was steadily going on emitting electrons although there were already in the tube so many electrons that just as many crowded back into the filament each second as the filament sent out. The filament was neither gaining nor losing electrons, although it was busy sending them out and welcoming them home again.

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 as many electrons crowding back into the filament as there were before and so the filament loses more than it gets back.

Suppose that, before the B-battery was connected to the plate, each tiny length of the filament was emitting 1000 electrons each second but was getting 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 from the filament of 100. Each second this tiny length 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 filament and plate and this current continues to flow as long as the A-battery and the B-battery do their work.

The negative terminal of the B-battery is connected to the filament.

Every time this battery pulls an electron from the plate its negative terminal shoves one out to the filament. You know from my third and fourth letters that electrons are carried through a battery from its positive to its negative terminal. You see, then, that there is the same stream of electrons through the B-battery as there is through the vacuum between filament and plate. This same stream pa.s.ses also through the wires which connect the battery to the tube. The path followed by the stream of electrons includes the wires, the vacuum and the battery in series. We call this path the "plate circuit."

We can connect a telephone receiver, or a current-measuring instrument, or any thing we wish which will pa.s.s a stream of electrons, so as to let this same stream of electrons pa.s.s through it also. All we have to do is to connect the instrument in series with the other parts of the plate circuit. I'll show you how in a minute, but just now I want you to understand that we have a stream of electrons, for I want to tell you how it may be controlled.

Suppose we use another battery and connect it between the grid and the filament so as to make the grid positive. That would mean connecting the positive terminal of the battery to the grid and the negative to the filament as shown by the C-battery of Fig. 8. This figure also shows a current-measuring instrument in the plate circuit.

What effect is this C-battery, or grid-battery, going to have on the current in the _plate circuit_? Making the grid positive makes it want electrons. It will therefore act just as we saw that the plate did and pull electrons across the vacuum towards itself.

[Ill.u.s.tration: Fig 8]

What happens then is something like this: Electrons are freed at the filament; the plate and the grid both call them and they start off in a rush. Some of them are stopped by the wires of the grid but most of them go on by to the plate. The grid is mostly open s.p.a.ce, you know, and the electrons move as fast as lightning. They are going too fast in the general direction of the grid to stop and look for its few and small wires.

When the grid is positive the grid helps the plate to call electrons away from the filament. Making the grid positive, therefore, increases the stream of electrons _between filament and plate_; that is, increases the current in the plate circuit.

We could get the same effect so far as concerns an increased plate current by using more batteries in series in the plate circuit so as to pull harder. But the grid is so close to the filament that a single battery cell in the grid circuit can call electrons so strongly that it would take several extra battery cells in the plate circuit to produce the same effect.

[Ill.u.s.tration: Fig 9]

If we reverse the grid battery, as in Fig. 9, so as to make the grid negative, then, instead of attracting electrons the grid repels them.

Nowhere near as many electrons will stream across to the plate when the grid says, "No, go back." The grid is in a strategic position and what it says has a great effect.

When there is no battery connected to the grid it has no possibility of influencing the electron stream which the plate is attracting to itself.

We say, then, that the grid is uncharged or is at "zero potential,"

meaning that it is zero or nothing in possibility. But when the grid is charged, no matter how little, it makes a change in the plate current.

When the grid says "Come on," even though very softly, it has as much effect on the electrons as if the plate shouted at them, and a lot of extra electrons rush for the plate. But when the grid whispers "Go back," many electrons which would otherwise have gone streaking off to the plate crowd back toward the filament. That's how the audion works.

There is an electron stream and a wonderfully sensitive way of controlling the stream.

LETTER 7

HOW TO MEASURE AN ELECTRON STREAM

(This letter may be omitted on the first reading.)

DEAR YOUTH:

If we are to talk about the audion and how its grid controls the current in the plate circuit we must know something of how to measure currents.

An electric current is a stream of electrons. We measure it by finding the rate at which electrons are traveling along through the circuit.

What do we mean by the word "rate?" You know what it means when a speedometer says twenty miles an hour. If the car should keep going just as it was doing at the instant you looked at the speedometer it would go twenty miles in the next hour. Its rate is twenty miles an hour even though it runs into a smash the next minute and never goes anywhere again except to the junk heap.

It's the same when we talk of electric currents. We say there is a current of such and such a number of electrons a second going by each point in the circuit. We don't mean that the current isn't going to change, for it may get larger or smaller, but we do mean that if the stream of electrons keeps going just as it is there will be such and such a number of electrons pa.s.s by in the next second.