Letters of a Radio-Engineer to His Son Part 7

When he started he stirred up his conscience and that opposed him.

n.o.body else was hindering his going. It was all brought about by his own actions. The opposition which he met was "self-induced." He was hindered at first by a self-induced effect of his own conscience. If he was a stream of electrons starting off to travel around the coil we would say that he was opposed by a self-induced e. m. f. And any path in which such an effect will be produced we say has "self-inductance."

Usually we shorten this term and speak of "inductance."

There is another way of looking at it. We know habits are hard to form and equally hard to break. It's hard to get electrons going around a coil and the self-inductance of a circuit tells us how hard it is. The harder it is the more self-inductance we say that the coil or circuit has. Of course, we need a unit in which to measure self-inductance. The unit is called the "henry." But that is more self-inductance than we can stand in most radio circuits, so we find it convenient to measure in smaller units called "mil-henries" which are thousandths of a henry.

You ought to know what a henry[4] is, if we are to use the word, but it isn't necessary just now to spend much time on it. The opposition which one's self-induced conscience offers depends upon how rapidly one starts. It's volts which make electrons move and so the conscience which opposes them will be measured in volts. Therefore we say that a coil has one henry of inductance when an electron stream which is increasing one ampere's worth each second stirs up in the coil a conscientious objection of one volt. Don't try to remember this now; you can come back to it later.

There is one more effect of inductance which we must know before we can get very far with our radio. Suppose an electron stream is flowing through a coil because a battery is driving the electrons along. Now let the battery be removed or disconnected. You'd expect the electron stream to stop at once but it doesn't. It keeps on for a moment because the electrons have got the habit.

[Ill.u.s.tration: Fig 28]

If you look again at Fig. 28 you will see what I mean. Suppose the switch is closed and a steady stream of electrons is flowing through the coil from _a_ to _b_. There will be no current in the other part of the coil. Now open the switch. There will be a motion of the needle of the current-measuring instrument, showing a momentary current.

The direction of this motion, however, shows that the momentary stream of electrons goes through the coil from _c_ to _d_.

Do you see what this means? The moment the battery is disconnected there is nothing driving the electrons in the part _ab_ and they slow down. Immediately, and just for an instant, a stream of electrons starts off in the part _cd_ in the same direction as if the battery was driving them along.

Now look again at Fig. 29. If the battery is suddenly disconnected there is a momentary rush of electrons in the same direction as the battery was driving them. Just as the self-inductance of a coil opposes the starting of a stream of electrons, so it opposes the stopping of a stream which is already going.

[Ill.u.s.tration: Fig 29]

So far we haven't said much about making an audion produce alternating e. m. f.'s and thus making it useful for radio-telephony. Before radio was possible all these things that I have just told you, and some more too, had to be known. It took hundreds of good scientists years of patient study and experiment to find out those ideas about electricity which have made possible radio-telephony.

Two of these ideas are absolutely necessary for the student of radio-communication. First: A condenser is a gap in a circuit where there are waiting-rooms for the electrons. Second: Electrons form habits. It's hard to get them going through a coil of wire, harder than through a straight wire, but after they are going they don't like to stop. They like it much less if they are going through a coil instead of a straight wire.

In my next letter I'll tell you what happens when we have a coil and a condenser together in a circuit.

[Footnote 4: The "henry" has nothing to do with a well-known automobile.

It was named after Joseph Henry, a professor years ago at Princeton University.]

LETTER 11

A "C-W" TRANSMITTER

DEAR SON:

[Ill.u.s.tration: Fig 28]

Let's look again at the coils of Fig. 28 which we studied in the last letter. I have reproduced them here so you won't have to turn back. When electrons start from _a_ towards _b_ there is a momentary stream of electrons from _d_ towards _c_. If the electron stream through _ab_ were started in the opposite direction, that is from _b_ to _a_ the induced stream in the coil _cd_ would be from _c_ towards _d_.

[Ill.u.s.tration: Fig 30]

It all reminds me of two boys with a hedge or fence between them as in Fig. 30. One boy is after the other. Suppose you were being chased; you know what you'd do. If your pursuer started off with a rush towards one end of the hedge you'd "beat it" towards the other. But if he started slowly and cautiously you would start slowly too. You always go in the opposite direction, dodging back and forth along the paths which you are wearing in the gra.s.s on opposite sides of the hedge. If he starts to the right and then slows up and starts back, you will start to your right, slow up, and start back. Suppose he starts at the center of the hedge.

First he dodges to the right, and then back through the center as far to the left, then back again and so on. You follow his every change.

[Ill.u.s.tration: Fig 31]

I am going to make a picture of what you two do. Let's start with the other fellow. He dodges or alternates back and forth. Some persons would say he "oscillates" back and forth in the same path. As he does so he induces you to move. I am on your side of the hedge with a moving-picture camera. My camera catches both of you. Fig. 31 shows the way the film would look if it caught only your heads. The white circle represents the tow-head on my side of the hedge and the black circle, young Brown who lives next door. Of course, the camera only catches you each time the shutter opens but it is easy to draw a complete picture of what takes place as time goes on. See Fig. 32.

[Ill.u.s.tration: Fig 32]

Now suppose you are an electron in coil _cd_ of Fig. 33 and "Brownie" is one in coil _ab_. Your motions are induced by his.

What's true of you two is true of all the other electrons. I have separated the coils a little in this sketch so that you can think of a hedge between. I don't know how one electron can affect another on the opposite side of this hedge but it can. And I don't know anything really about the hedge, which is generally called "the ether." The hedge isn't air. The effect would be the same if the coils were in a vacuum. The "ether" is just a name for whatever is left in the s.p.a.ce about us when we have taken out everything which we can see or feel--every molecule, every proton and every electron.

[Ill.u.s.tration: Fig 33]

Why and how electrons can affect one another when they are widely separated is one of the great mysteries of science. We don't know any more about it than about why there are electrons. Let's accept it as a fundamental fact which we can't as yet explain.

[Ill.u.s.tration: Fig 34]

And now we can see how to make an audion produce an alternating current or as we sometimes say "make an audion oscillator." We shall set up an audion with its A-battery as in Fig. 34. Between the grid and the filament we put a coil and a condenser. Notice that they are in parallel, as we say. In the plate-filament circuit we connect the B-battery and a switch, _S_, and another coil. This coil in the plate circuit of the audion we place close to the other coil so that the two coils are just like the coils _ab_ and _cd_ of which I have been telling you. The moment any current flows in coil _ab_ there will be a current flow in the coil _cd_. (An induced electron stream.) Of course, as long as the switch in the B-battery is open no current can flow.

The moment the switch _S_ is closed the B-battery makes the plate positive with respect to the filament and there is a sudden surge of electrons round the plate circuit and through the coil from _a_ to _b_. You know what that does to the coil _cd_. It induces an electron stream from _d_ towards _c_. Where do these electrons come from? Why, from the grid and the plate 1 of the condenser. Where do they go? Most of them go to the waiting-room offered by plate 2 of the condenser and some, of course, to the filament. What is the result? The grid becomes positive and the filament negative.

[Ill.u.s.tration: Fig 35]

This is the crucial moment in our study. Can you tell me what is going to happen to the stream of electrons in the plate circuit? Remember that just at the instant when we closed the switch the grid was neither positive nor negative. We were at the point of zero volts on the audion characteristic of Fig. 35. When we close the switch the current in the plate circuit starts to jump from zero mil-amperes to the number of mil-amperes which represents the point where Zero Volt St. crosses Audion Characteristic. But this jump in plate current makes the grid positive as we have just seen. So the grid will help the plate call electrons and that will make the current in the plate circuit still larger, that is, result in a larger stream of electrons from _a_ to _b_.

This increase in current will be matched by an increased effect in the coil _cd_, for you remember how you and "Brownie" behaved. And that will pull more electrons away from plate 1 of the condenser and send them to the waiting-room of 2. All this makes the grid more positive and so makes it call all the more effectively to help the plate move electrons.

[Ill.u.s.tration: Pl. V.--Variometer (top) and Variable Condenser (bottom) of the General Radio Company. Voltmeter and Ammeter of the Weston Instrument Company.]

We "started something" that time. It's going on all by itself. The grid is getting more positive, the plate current is getting bigger, and so the grid is getting more positive and the plate current still bigger. Is it ever going to stop? Yes. Look at the audion characteristic. There comes a time when making the grid a little more positive won't have any effect on the plate-circuit current. So the plate current stops increasing.

There is nothing now to keep pulling electrons away from plate 1 and crowding them into waiting-room 2. Why shouldn't the electrons in this waiting-room go home to that of plate 1? There is now no reason and so they start off with a rush.

Of course, some of them came from the grid and as fast as electrons get back to the grid it becomes less and less positive. As the grid becomes less and less positive it becomes less and less helpful to the plate.

If the grid doesn't help, the plate alone can't keep up this stream of electrons. All the plate can do by itself is to maintain the current represented by the intersection of zero volts and the audion characteristic. The result is that the current in the plate circuit, that is, of course, the current in coil _ab_, becomes gradually less.

About the time all the electrons, which had left the grid and plate 1 of the condenser, have got home the plate current is back to the value corresponding to _E_{C}_=_0_.

The plate current first increases and then decreases, but it doesn't stop decreasing when it gets back to zero-grid value. And the reason is all due to the habit forming tendencies of electrons in coils. To see how this comes about, let's tell the whole story over again. In other words let's make a review and so get a sort of flying start.

[Ill.u.s.tration: Fig 34]

When we close the battery switch, _S_ in Fig. 34, we allow a current to flow in the plate circuit. This current induces a current in the coil _cd_ and charges the condenser which is across it, making plate 1 positive and plate 2 negative. A positive grid helps the plate so that the current in the plate circuit builds up to the greatest possible value as shown by the audion characteristic. That's the end of the increase in current. Now the condenser discharges, sending electrons through the coil _cd_ and making the grid less positive until finally it is at zero potential, that is neither positive nor negative.

While the condenser is discharging the electrons in the coil _cd_ get a habit of flowing from _c_ toward _d_, that is from plate 2 to plate 1. If it wasn't for this habit the electron stream in _cd_ would stop as soon as the grid had reduced to zero voltage.

Because of the habit, however, a lot of electrons that ought to stay on plate 2 get hurried along and land on plate 1. It is a little like the old game of "crack the whip." Some electrons get the habit and can't stop quickly enough so they go tumbling into waiting-room 1 and make it negative.

That means that the condenser not only discharges but starts to get charged in the other direction with plate 1 negative and plate 2 positive. The grid feels the effect of all this, because it gets extra electrons if plate 1 gets them. In fact the voltage effective between grid and filament is always the voltage between the plates of the condenser.

The audion characteristic tells us what is the result. As the grid becomes negative it opposes the plate, shooing electrons back towards the filament and reducing the plate current still further. But you have already seen in my previous letter what happens when we reduce the current in coil _ab_. There is then induced in coil _cd_ an electron stream from _c_ to _d_. This induced current is in just the right direction to send more electrons into waiting-room 1 and so to make the grid still more negative. And the more negative the grid gets the smaller becomes the plate current until finally the plate current is reduced to zero. Look at the audion characteristic again and see that making the grid sufficiently negative entirely stops the plate current.

When the plate current stops, the condenser in the grid circuit is charged, with plate 1 negative and 2 positive. It was the plate current which was the main cause of this change for it induced the charging current in coil _cd_. So, when the plate current becomes zero there is nothing to prevent the condenser from discharging.