Chapter XI: Letter 10: Condensers and Coils

DEAR SON:

In the last letter we learned of an alternating e. m. f. The way of producing it, which I described, is very crude and I want to tell how to make the audion develop an alternating e. m. f. for itself. That is what the audion does in the transmitting set of a radio telephone. But an audion can't do it all alone. It must have associated with it some coils and a condenser. You know what I mean by coils but you have yet to learn about condensers.

A condenser is merely a gap in an otherwise conducting circuit. It's a gap across which electrons cannot pass so that if there is an e. m. f. in the circuit, electrons will be very plentiful on one side of the gap and scarce on the other side. If there are to be many electrons waiting beside the gap there must be room for them. For that reason we usually provide waiting-rooms for the electrons on each side of the gap. Metal plates or sheets of tinfoil serve nicely for this purpose. Look at Fig. 25. You see a battery and a circuit which would be conducting except for the gap at _C_. On each side of the gap there is a sheet of metal. The metal sheets may be separated by air or mica or paraffined paper. The combination of gap, plates, and whatever is between, provided it is not conducting, is called a condenser.

Let us see what happens when we connect a battery to a condenser as in the figure. The positive terminal of the battery calls electrons from one plate of the condenser while the negative battery-terminal drives electrons away from itself toward the other plate of the condenser. One plate of the condenser, therefore, becomes positive while the other plate becomes negative.

You know that this action of the battery will go on until there are so many electrons in the negative plate of the condenser that they prevent the battery from adding any more electrons to that plate. The same thing happens at the other condenser plate. The positive terminal of the battery calls electrons away from the condenser plate which it is making positive until so many electrons have left that the protons in the atoms of the plate are calling for electrons to stay home just as loudly and effectively as the positive battery-terminal is calling them away.

When both these conditions are reached--and they are both reached at the same time--then the battery has to stop driving electrons around the circuit. The battery has not enough e. m. f. to drive any more electrons. Why? Because the condenser has now just enough e. m. f. with which to oppose the battery.

It would be well to learn at once the right words to use in describing this action. We say that the battery sends a "charging current" around its circuit and "charges the condenser" until it has the same e. m. f. When the battery is first connected to the condenser there is lots of space in the waiting-rooms so there is a great rush or surge of electrons into one plate and away from the other. Just at this first instant the charging current, therefore, is large but it decreases rapidly, for the moment electrons start to pile up on one plate of the condenser and to leave the other, an e. m. f. builds up on the condenser. This e. m. f., of course, opposes that of the battery so that the net e. m. f. acting to move electrons round the circuit is no longer that of the battery, but is the difference between the e. m. f. of the battery and that of the condenser. And so, with each added electron, the e. m. f. of the condenser increases until finally it is just equal to that of the battery and there is no net e. m. f. to act.

What would happen if we should then disconnect the battery? The condenser would be left with its extra electrons in the negative plate and with its positive plate lacking the same number of electrons. That is, the condenser would be left charged and its e. m. f. would be of the same number of volts as the battery.

Now suppose we connect a short wire between the plates of the condenser as in Fig. 26. The electrons rush home from the negative plate to the positive plate. As fast as electrons get home the e. m. f. decreases. When they are all back the e. m. f. has been reduced to zero. Sometimes we say that "the condenser discharges." The "discharge current" starts with a rush the moment the conducting path is offered between the two plates. The e. m. f. of the condenser falls, the discharge current grows smaller, and in a very short time the condenser is completely discharged.

That's what happens when there is a short conducting path for the discharge current. If that were all that could happen I doubt if there would be any radio communication to-day. But if we connect a coil of wire between two plates of a charged condenser, as in Fig. 27, then something of great interest happens. To understand you must know something more about electron streams.

Suppose we should wind a few turns of wire on a cylindrical core, say on a stiff cardboard tube. We shall use insulated wire. Now start from one end of the coil, say _a_, and follow along the coiled wire for a few turns and then scratch off the insulation and solder onto the coil two wires, _b_, and _c_, as shown in Fig. 28. The further end of the coil we shall call _d_. Now let's arrange a battery and switch so that we can send a current through the part of the coil between _a_ and _b_. Arrange also a current-measuring instrument so as to show if any current is flowing in the part of the coil between _c_ and _d_. For this purpose we shall use a kind of current-measuring instrument which I have not yet explained. It is different from the hot-wire type described in Letter 7 for it will show in which direction electrons are streaming through it.

The diagram of Fig. 28 indicates the apparatus of our experiment. When we close the switch, _S_, the battery starts a stream of electrons from _a_ towards _b_. Just at that instant the needle, or pointer, of the current instrument moves. The needle moves, and thus shows a current in the coil _cd_; but it comes right back again, showing that the current is only momentary. Let's say this again in different words. The battery keeps steadily forcing electrons through the circuit _ab_ but the instrument in the circuit _cd_ shows no current in that circuit except just at the instant when current starts to flow in the neighboring circuit _ab_.

One thing this current-measuring instrument tells us is the direction of the electron stream through itself. It shows that the momentary stream of electrons goes through the coil from _d_ to _c_, that is in the opposite direction to the stream in the part _ab_.

Now prepare to do a little close thinking. Read over carefully all I have told you about this experiment. You see that the moment the battery starts a stream of electrons from _a_ towards _b_, something causes a momentary, that is a temporary, movement of electrons from _d_ to _c_. We say that starting a stream of electrons from _a_ to _b_ sets up or "induces" a stream of electrons from _d_ to _c_.

What will happen then if we connect the battery between _a_ and _d_ as in Fig. 29? Electrons will start streaming away from _a_ towards _b_, that is towards _d_. But that means there will be a momentary stream from _d_ towards _c_, that is towards _a_. Our stream from the battery causes this oppositely directed stream. In the usual words we say it "induces" in the coil an opposing stream of electrons. This opposing stream doesn't last long, as we saw, but while it does last it hinders the stream which the battery is trying to establish.

The stream of electrons which the battery causes will at first meet an opposition so it takes a little time before the battery can get the full-sized stream of electrons flowing steadily. In other words a current in a coil builds up slowly, because while it is building up it induces an effect which opposes somewhat its own building up.

Did you ever see a small boy start off somewhere, perhaps where he shouldn't be going, and find his conscience starting to trouble him at once. For a time he goes a little slowly but in a moment or two his conscience stops opposing him and he goes on steadily at his full pace. When he started he stirred up his conscience and that opposed him. Nobody 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.

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.

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.]