Suppose we take batteries which aren’t going to be injured by being made to work–storage batteries will do nicely–and connect them in series as in Fig. 13. When batteries are in series they act like a single stronger battery, one whose e. m. f. is the sum of the e. m. f.’s of the separate batteries. Connect these batteries to a long fine wire as in Fig. 14. There is a stream of electrons along this wire. Next connect the negative terminal of the standard cell to the negative terminal of the storage batteries, that is, brace their feet against each other. Then connect a wire to the positive terminal of the standard cell. This wire acts just like a long arm sticking out from the positive plate of this cell.
Touch the end of the wire, which ispof Fig. 14,61to some point asaon the fine wire. Now what do we have? Right ata, of course, there are some free electrons and they hear the calls of both batteries. If the standard battery,Sof the figure, calls the stronger they go to it. In that case move the endpnearer the positive plate of the batteryB, so that it will have a chance to exert a stronger pull. Suppose we try atcand find the batteryBis there the stronger. Then we can move back to some point, sayb, where the pulls are equal.
To make a test like this we put a sensitive current-measuring instrument in the wire which leads from the positive terminal of the standard cell. We also use a long fine wire so that there can never be much of an electron stream anyway. When the pulls are equal there will be no current through this instrument.
As soon as we find out where the proper setting is we can replaceSby some other battery, sayX, which we wish to compare withS. We find the setting for that battery in the same way as we just did forS. Suppose it is atdin Fig. 14 while the setting forSwas atb. We can see at once thatXis stronger thanS. The question, however, is how much stronger.
Perhaps it would be better to try to answer this question by talking about e. m. f.’s. It isn’t fair to speak only of the positive plate which calls, we must speak also of the negative plate which is shooing electrons away from itself. The idea of e. m. f. takes care of both these actions. The steady stream of62electrons in the fine wire is due to the e. m. f. of the batteryB, that is to the pull of the positive terminal and the shove of the negative.
If the wire is uniform, that is the same throughout its length, then each inch of it requires just as much e. m. f. as any other inch. Two inches require twice the e. m. f. which one inch requires. We know how much e. m. f. it takes to keep the electron stream going in the part of the wire fromntob. It takes just the e. m. f. of the standard cell,S, because when that had its feet braced atnit pulled just as hard atbas did the big batteryB.
Suppose the distancentod(usually writtennd) is twice as great as that fromntob(nb). That means that batteryXhas twice the e. m. f. of batteryS. You remember thatXcould exert the same force through the length of wirend, as could the large battery. That is twice what cellScan do. Therefore if we know how many volts to call the e. m. f. of the standard cell we can say thatXhas an e. m. f. of twice as many volts.
If we measured dry batteries this way we should find that they each had an e. m. f. of about 1.46 volts. A storage battery would be found to have about 2.4 volts when fully charged and perhaps as low as 2.1 volts when we had run it for a while.
That is the way in which to compare batteries and to measure their e. m. f.’s, but you see it takes a lot of time. It is easier to use a “voltmeter” which is an instrument for measuring e. m. f.’s. Here is how one could be made.
63First there is made a current-measuring instrument which is quite sensitive, so that its pointer will show a deflection when only a very small stream of electrons is passing through the instrument. We could make one in the same way as we made the ammeter of the last letter but there are other better ways of which I’ll tell you later. Then we connect a good deal of fine wire in series with the instrument for a reason which I’ll tell you in a minute. The next and last step is to calibrate.
We know how many volts of e. m. f. are required to keep going the electron stream betweennandb–we know that from the e. m. f. of our standard cell. Suppose then that we connect this new instrument, which we have just made, to the wire atnandbas in Fig. 15. Some of the electrons atnwhich are so anxious to get away from the negative plate of batteryBcan now travel as far asbthrough the wire of the new instrument. They do so and the pointer swings around to some new position. Opposite that we mark the number of volts which the standard battery told us there was betweennandb.
If we move the end of the wire frombtodthe pointer will take a new position. Opposite this we mark twice the number of volts of the standard cell. We can run it to a pointewhere the distanceneis one-halfnb, and mark our scale with half the number of volts of the standard cell, and so on for other64positions along the wire. That’s the way we calibrate a sensitive current-measuring instrument (with its added wire, of course) so that it will read volts. It is now a voltmeter.
If we connect a voltmeter to the batteryXas in Fig. 16 the pointer will tell us the number of volts in the e. m. f. ofX, for the pointer will take the same position as it did when the voltmeter was connected betweennandd.
There is only one thing to watch out for in all this. We must be careful that the voltmeter is so made that it won’t offer too easy a path for electrons to follow. We only want to find how hard a battery can pull an electron, for that is what we mean by e. m. f. Of course, we must let a small stream of electrons flow through the voltmeter so as to make the pointer move. That is why voltmeters of this kind are made out of a long piece of fine wire or else have a coil of fine wire in series with the current-measuring part. The fine wire makes a long and narrow path for the electrons and so there can be only a small stream. Usually we describe this condition by saying that a voltmeter has a high resistance.
Fine wires offer more resistance to electron streams than do heavy wires of the same length. If a wire is the same diameter all along, the longer the length of it which we use the greater is the resistance which is offered to an electron stream.
65You will need to know how to describe the resistance of a wire or of any part of an electric circuit. To do so you tell how many “ohms” of resistance it has. The ohm is the unit in which we measure the resistance of a circuit to an electron stream.
I can show you what an ohm is if I tell you a simple way to measure a resistance. Suppose you have a wire or coil of wire and want to know its resistance. Connect it in series with a battery and an ammeter as shown in Fig. 17. The same electron stream passes through all parts of this circuit and the ammeter tells us what this stream is in amperes. Now connect a voltmeter to the two ends of the coil as shown in the figure. The voltmeter tells in volts how much e. m. f. is being applied to force the current through the coil. Divide the number of volts by the number of amperes and the quotient (answer) is the number of ohms of resistance in the coil.
Suppose the ammeter shows a current of one ampere and the voltmeter an e. m. f. of one volt. Then dividing 1 by 1 gives 1. That means that the coil has a resistance of one ohm. It also means one ohm is such a resistance that one volt will send through it a current of one ampere. You can get lots of meaning out of this. For example, it means also66that one volt will send a current of one ampere through a resistance of one ohm.
How many ohms would the coil have if it took 5 volts to send 2 amperes through it. Solution: Divide 5 by 2 and you get 2.5. Therefore the coil would have a resistance of 2.5 ohms.
Try another. If a coil of resistance three ohms is carrying two amperes what is the voltage across the terminals of the coil? For 1 ohm it would take 1 volt to give a current of 1 ampere, wouldn’t it? For 3 ohms it takes three times as much to give one ampere. To give twice this current would take twice 3 volts. That is, 2 amperes in 3 ohms requires 2x3 volts.
Here’s one for you to try by yourself. If an e. m. f. of 8 volts is sending current through a resistance of 2 ohms, how much current is flowing? Notice that I told the number of ohms and the number of volts, what are you going to tell? Don’t tell just the number; tell how many and what.
My Dear Young Student:
Although there is much in Letters 7 and 8 which it is well to learn and to think about, there are only three of the ideas which you must have firmly grasped to get the most out of this letter which I am now going to write you about the audion.
First: Electric currents are streams of electrons. We measure currents in amperes. To measure a current we may connect into the circuit an ammeter.
Second: Electrons move in a circuit when there is an electron-moving-force, that is an electromotive force or e. m. f. We measure e. m. f.’s in volts. To measure an e. m. f. we connect a voltmeter to the two points between which the e. m. f. is active.
Third: What current any particular e. m. f. will cause depends upon the circuit in which it is active. Circuits differ in the resistance which they offer to e. m. f.’s. For any particular e. m. f. (that is for any given e. m. f.) the resulting current will be smaller the greater the resistance of the circuit. We measure resistance in ohms. To measure it we find the quotient of the number of volts applied to the circuit by the number of amperes which flow.
In my sixth letter I told you something of how the audion works. It would be worth while to read again that letter. You remember that the current in the68plate circuit can be controlled by the e. m. f. which is applied to the grid circuit. There is a relationship between the plate current and the grid voltage which is peculiar or characteristic to the tube. So we call such a relationship “a characteristic.” Let us see how it may be found and what it will be.
Connect an ammeter in the plate- or B-circuit, of the tube so as to measure the plate-circuit current. You will find that almost all books use the letter “I” to stand for current. The reason is that scientists used to speak of the “intensity of an electric current” so that “I” really stands for intensity. We useIto stand for something more than the word “current.” It is our symbol for whatever an ammeter would read, that is for the amount of current.
Another convenience in symbols is this: We shall frequently want to speak of the currents in several different circuits. It saves time to use another letter along with the letterIto show the circuit to which we refer. For example, we are going to talk about the current in the B-circuit of the audion, so we call that currentIB. We write the letterBbelow the line on whichIstands. That is why we say theBis subscript, meaning “written below.” When you are reading to yourself be sure to readIBas “eye-bee” or else as “eye-subscript-bee.”IBtherefore will stand for the number of amperes in the69plate circuit of the audion. In the same wayIAwould stand for the current in the filament circuit.
We are going to talk about e. m. f.’s also. The letter “E” stands for the number of volts of e. m. f. in a circuit. In the filament circuit the battery hasEAvolts. In the plate circuit the e. m. f. isEBvolts. If we put a battery in the grid circuit we can letECrepresent the number of volts applied to the grid-filament or C-circuit.
The characteristic relation which we are after is one between grid voltage, that isEC, and plate current, that isIB. So we call it theEC–IBcharacteristic. The dash between the letters is not a subtraction sign but merely a dash to separate the letters. Now we’ll find the “ee-see-eye-bee” characteristic.
Connect some small dry cells in series for use in the grid circuit. Then connect the filament to the middle cell as in Fig. 19. Take the wire which comes from the grid and put a battery clip on it, then you can connect the grid anywhere you want along this series of batteries. See Fig. 18. In the figure this movable clip is represented by an arrow head. You can see that if it is atathe battery will make the grid positive. If it is moved tobthe grid will be more positive. On the other hand if the clip is atothere will be no e. m. f. applied to the grid. If it is atcthe grid will be made negative.
Between grid and filament there is placed a voltmeter which will tell how much e. m. f. is applied to the grid, that is, tell the value ofEC, for any position whatever of the clip.
70We shall start with the filament heated to a deep red. The manufacturers of the audion tell the purchaser what current should flow through the filament so that there will be the proper emission of electrons. There are easy ways of finding out for one’s self but we shall not stop to describe them. The makers also tell how many volts to apply to the plate, that is what valueEBshould have. We could find this out also for ourselves but we shall not stop to do so.
Now we set the battery clip so that there is no voltage applied to the grid; that is, we start withECequal to zero. Then we read the ammeter in the plate circuit to find the value ofIBwhich corresponds to this condition of the grid.
Next we move the clip so as to make the grid as positive as one battery will make it, that is we move the clip toain Fig. 19. We now have a different value ofECand will find a different value ofIBwhen we read the ammeter. Next move the clip to apply two batteries to the grid. We get a new pair of values forECandIB, gettingECfrom the voltmeter andIBfrom the ammeter. As we continue in this way, increasingEC, we find that the currentIBincreases71for a while and then after we have reached a certain value ofECthe currentIBstops increasing. Adding more batteries and making the grid more positive doesn’t have any effect on the plate current.
Before I tell you why this happens I want to show you how to make a picture of the pairs of values ofECandIBwhich we have been reading on the voltmeter and ammeter.
Imagine a city where all the streets are at right angles and the north and south streets are called streets and numbered while the east and west thorofares are called avenues. I’ll draw the map as in Fig. 20. Right through the center of the city goes Main Street. But the people who laid out the roads were mathematicians and instead of calling it Main Street they called it “Zero Street.” The first street east of Zero St. we should have called “East First Street” but they called it “Positive 1 St.” and the72next beyond “Positive 2 St.,” and so on. West of the main street they called the first street “Negative 1 St.” and so on.
When they came to name the avenues they were just as precise and mathematical. They called the main avenue “Zero Ave.” and those north of it “Positive 1 Ave.,” “Positive 2 Ave.” and so on. Of course, the avenues south of Zero Ave. they called Negative.
The Town Council went almost crazy on the subject of numbering; they numbered everything. The silent policeman which stood at the corner of “Positive 2 St.” and “Positive 1 Ave.” was marked that way. Half way between Positive 2 St. and Positive 3 St. there was a garage which set back about two-tenths of a block from Positive 1 Ave. The Council numbered it and called it “Positive 2.5 St. and Positive 1.2 Ave.” Most of the people spoke of it as “Plus 2.5 St. and Plus 1.2 Ave.”
Sometime later there was an election in the city and a new Council was elected. The members were mostly young electricians and the new Highway Commissioner was a radio enthusiast. At the first meeting the Council changed the names of all the avenues to “Mil-amperes”[3]and of all the streets to “Volts.”
Then the Highway Commissioner who had just been taking a set of voltmeter and ammeter readings on an audion moved that there should be a new73road known as “Audion Characteristic.” He said the road should pass through the following points:
Zero Volt and Plus 1.0 Mil-amperePlus 2.0 Volts and Plus 1.7 Mil-amperesPlus 4.0 Volts and Plus 2.6 Mil-amperesPlus 6.0 Volts and Plus 3.4 Mil-amperesPlus 8.0 Volts and Plus 4.3 Mil-amperes
Zero Volt and Plus 1.0 Mil-amperePlus 2.0 Volts and Plus 1.7 Mil-amperesPlus 4.0 Volts and Plus 2.6 Mil-amperesPlus 6.0 Volts and Plus 3.4 Mil-amperesPlus 8.0 Volts and Plus 4.3 Mil-amperes
And so on. Fig. 21 shows the new road.
One member of the Council jumped up and said “But what if the grid is made negative?” The Commissioner had forgotten to see what happened so he went home to take more readings.
He shifted the battery clip along, starting atcof74Fig. 22. At the next meeting of the Council he brought in the following list of readings and hence of points on his proposed road.
Then he showed the other members of the Council on the map of Fig. 23 how the Audion Characteristic would look.
There was considerable discussion after that and it appeared that different designs and makes of audions would have different characteristic curves. They all had the same general form of curve but they would pass through different sets of points depending upon the design and upon the B-battery voltage. It was several meetings later, however, before they found out what effects were due to the form of the curve. Right after this they found that they could get much better results with their radio sets.
Now look at the audion characteristic. Making the grid positive, that is going on the positive side of the zero volts in our map, makes the plate current75larger. You remember that I told you in Letter 6 how the grid, when positive, helped call electrons away from the filament and so made a larger stream of electrons in the plate circuit. The grid calls electrons away from the filament. It can’t call them out of it; they have to come out themselves as I explained to you in the fifth letter.
You can see that as we make the grid more and more positive, that is, make it call louder and louder, a condition will be reached where it won’t do it any good to call any louder, for it will already be getting all the electrons away from the filament just as fast as they are emitted. Making the grid more positive after that will not increase the plate current any. That’s why the characteristic flattens off as you see at high values of grid voltage.
The arrangement which we pictured in Fig. 22 for76making changes in the grid voltage is simple but it doesn’t let us change the voltage by less than that of a single battery cell. I want to show you a way which will. You’ll find it very useful to know and it is easily understood for it is something like the arrangement of Fig. 14 in the preceding letter.
Connect the cells as in Fig. 24 to a fine wire. About the middle of this wire connect the filament. As before use a clip on the end of the wire from the grid. If the grid is connected toain the figure there is applied to the grid circuit that part of the e. m. f. of the battery which is active in the length of wire betweenoanda. The pointais nearer the positive plate of the battery than is the pointo. So the grid will be positive and the filament negative.
On the other hand, if the clip is connected atbthe grid will be negative with respect to the filament. We can, therefore, make the grid positive or negative depending on which side ofowe connect the clip. How large the e. m. f. is which will be applied to the grid depends, of course, upon how far away fromothe clip is connected.
Suppose you took the clip in your hand and slid it along in contact with the wire, first fromotoa77and then back again throughotoband so on back and forth. You would be making the gridalternatelypositive and negative, wouldn’t you? That is, you would be applying to the grid an e. m. f. which increases to some positive value and then, decreasing to zero,reverses, and increases just as much, only to decrease to zero, where it started. If you do this over and over again, taking always the same time for one round trip of the clip you will be impressing on the grid circuit an “alternating e. m. f.”
What’s going to happen in the plate circuit? When there is no e. m. f. applied to the grid circuit, that is when the grid potential (possibilities) is zero, there is a definite current in the plate circuit. That current we can find from our characteristic of Fig. 23 for it is where the curve crosses Zero Volts. As the grid becomes positive the current rises above this value. When the grid is made negative the current falls below this value. The current,IB, then is made alternately greater and less than the current whenECis zero.
You might spend a little time thinking over this, seeing what happens when an alternating e. m. f. is applied to the grid of an audion, for that is going to be fundamental to our study of radio.
[3]A mil-ampere is a thousandth of an ampere just as a millimeter is a thousandth of a meter.
A mil-ampere is a thousandth of an ampere just as a millimeter is a thousandth of a meter.
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 atC. 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. The79combination 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 words80to 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 home81the 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, saya, and follow along the coiled wire for a few turns and then scratch off the insulation and solder onto the coil two wires,b, andc, as shown in Fig. 28. The further end of the coil we shall calld. Now let’s arrange a battery and switch so that we can send a current through the part of the coil betweenaandb. Arrange also a current-measuring instrument so as to show if any current is flowing in the part of the coil betweencandd. For this purpose we shall use a kind of current-measuring82instrument 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 fromatowardsb. Just at that instant the needle, or pointer, of the current instrument moves. The needle moves, and thus shows a current in the coilcd; 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 circuitabbut the instrument in the circuitcdshows no current in that circuit except just at the instant when current starts to flow in the neighboring circuitab.
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 fromdtoc, that is in the opposite direction to the stream in the partab.
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 fromatowardsb, something causes a momentary, that is a temporary, movement of electrons fromdtoc. We say that starting a83stream of electrons fromatobsets up or “induces” a stream of electrons fromdtoc.
What will happen then if we connect the battery betweenaanddas in Fig. 29? Electrons will start streaming away fromatowardsb, that is towardsd. But that means there will be a momentary stream fromdtowardsc, that is towardsa. 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 he84met 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 stream85which 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 fromatob. 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 fromctod.
Do you see what this means? The moment the battery is disconnected there is nothing driving the electrons in the partaband they slow down. Immediately, and just for an instant, a stream of electrons starts off in the partcdin the same direction as if the battery was driving them along.
86Now 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.
[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.
The “henry” has nothing to do with a well-known automobile. It was named after Joseph Henry, a professor years ago at Princeton University.
Dear Son:
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 fromatowardsbthere is a momentary stream of electrons fromdtowardsc. If the electron stream throughabwere started in the opposite direction, that is frombtoathe induced stream in the coilcdwould be fromctowardsd.
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 off88with 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 grass 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.
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. As89he 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.
Now suppose you are an electron in coilcdof Fig. 33 and “Brownie” is one in coilab. 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 space about us when we have taken out everything90which we can see or feel–every molecule, every proton and every electron.
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.
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 coilsabandcdof which I have been telling you. The moment any current flows in coilabthere will be a current flow in the coilcd. (An induced electron stream.) Of course, as long as the switch in the B-battery is open no current can flow.
The moment the switchSis closed the B-battery makes the plate positive with respect to the filament and there is a sudden surge of electrons round the91plate circuit and through the coil fromatob. You know what that does to the coilcd. It induces an electron stream fromdtowardsc. 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.
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 fromatob.
This increase in current will be matched by an increased effect in the coilcd, for you remember92how 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.
Pl. V.–Variometer (top) and Variable Condenser (bottom) of the General Radio Company.Voltmeter and Ammeter of the Weston Instrument Company.
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 coilab,93becomes 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 toEC=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.