Letters of a Radio-Engineer to His Son Part 2

LETTER 4

THE BATTERIES IN YOUR RADIO SET

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

MY DEAR YOUNG MAN:

You will need several batteries when you come to set up your radio receiver but you won't use such clumsy affairs as the gravity cell which I described in my last letter. Some of your batteries will be dry batteries of the size used in pocket flash lights.

These are not really dry, for between the plates they are filled with a moist paste which is then sealed in with wax to keep it from drying out or from spilling. Instead of zinc and copper these batteries use zinc and carbon. No gla.s.s jar is needed, for the zinc is formed into a jar shape. In this is placed the paste and in the center of the paste a rod or bar of carbon. The paste doesn't contain sulphuric acid, but instead has in it a stuff called sal ammoniac; that is, ammonium chloride.

The battery, however, acts very much like the one I described in my last letter. Ions of zinc leave the zinc and wander into the moist paste.

These ions are positive, just as in the case of the gravity battery. The result is that the electrons which used to a.s.sociate with a zinc ion to form a zinc atom are left in the zinc plate. That makes the zinc negative for it has more electrons than protons. The zinc ions take the place of the positive ions which are already in the paste. The positive ions which originally belonged with the paste, therefore, move along to the carbon rod and there get some electrons. Taking electrons away from the carbon leaves it with too many protons; that is, leaves it positive.

In the little flash light batteries, therefore, you will always find that the round carbon rod, which sticks out of the center, is positive and the zinc casing is negative.

The trouble with the battery like the one I used to make is that the zinc plate wastes away. Every time a zinc ion leaves it that means that the greater part of an atom is gone. Then when the two electrons which were left behind get a chance to start along a copper wire toward the positive plate of the battery there goes the rest of the atom. After a while there is no more zinc plate. It is easy to see what has happened.

All the zinc has gone into solution or been "eaten away" as most people say. Dry batteries, however, don't stop working because the zinc gets used up, but because the active stuff in the paste, the ammonium chloride, is changed into something else.

There's another kind of battery which you will need to use with your radio set; that is the storage battery. Storage batteries can be used over and over again if they are charged between times and will last for a long time if properly cared for. Then too, they can give a large current, that is, a big swift-moving stream of electrons. You will need that when you wish to heat the filament of the audion in your receiving set.

The English call our storage batteries by the name "acc.u.mulators." I don't like that name at all, but I don't like our name for them nearly as well as I do the name "reversible batteries." n.o.body uses this last name because it's too late to change. Nevertheless a storage battery is reversible, for it will work either way at an instant's notice.

A storage battery is something like a boy's wagon on a hill side. It will run down hill but it can be pushed up again for another descent.

You can use it to send a stream of electrons through a wire from its negative plate to its positive plate. Then if you connect these plates to some other battery or to a generator, (that is, a dynamo) you can make a stream of electrons go in the other direction. When you have done so long enough the battery is charged again and ready to discharge.

I am not going to tell you very much about the storage battery but you ought to know a little about it if you are to own and run one with your radio set. When it is all charged and ready to work, the negative plate is a lot of soft spongy lead held in place by a frame of harder lead.

The positive plate is a lead frame with small squares which are filled with lead peroxide, as it is called. This is a substance with molecules formed of one lead atom and two oxygen atoms. Why the chemists call it lead peroxide instead of just lead oxide I'll tell you some other time, but not in these letters.

Between the two plates is a wood separator to keep pieces of lead from falling down between and touching both plates. You know what would happen if a piece of metal touched both plates. There would be a short circuit, that is, a sort of a short cut across lots by which some of the electrons from the negative plate could get to the positive plate without going along the wires which we want them to travel. That's why there are separators.

The two plates are in a jar of sulphuric acid solution. The sulphuric acid has molecules which split up in solution, as you remember, into hydrogen ions and the ions which we called "sulphate." In my gravity battery the sulphate ions used to coax the zinc ions away into the solution. In the storage battery on the other hand the sulphate ions can get to most of the lead atoms because the lead is so spongy. When they do, they form lead sulphate right where the lead atoms are. They don't really need whole lead atoms, because they have two more electrons than they deserve, so there are two extra electrons for every molecule of lead sulphate which is formed. That's why the spongy lead plate is negative.

The lead sulphate won't dissolve, so it stays there on the plate as a whitish coating. Now see what that means. What are the hydrogen ions going to do? As long as there was sulphuric acid in the water there was plenty of sulphate ions for them to a.s.sociate with as often as they met; and they would meet pretty often. But if the sulphate ions get tied up with the lead of the plate there will be too many hydrogen ions left in the solution. Now what are the hydrogen ions to do? They are going to get as far away from each other as they can, for they are nothing but protons; and protons don't like to a.s.sociate. They only stayed around in the first place because there was always plenty of sulphate ions with whom they liked to play.

When the hydrogen ions try to get away from each other they go to the other plate of the battery, and there they will get some electrons, if they have to steal in their turn.

I won't try to tell you all that happens at the other plate. The hydrogen ions get the electrons which they need, but they get something more. They get some of the oxygen away from the plate and so form molecules of water. You remember that water molecules are made of two atoms of hydrogen and one of oxygen. Meanwhile, the lead atoms, which have lost their oxygen companions, combine with some of the sulphate ions which are in that neighborhood. During the mix-up electrons are carried away from the plate and that leaves it positive.

The result of all this is a little lead sulphate on each plate, a negative plate where the spongy lead was, and a positive plate where the lead peroxide was.

Notice very carefully that I said "a little lead sulphate on each plate." The sort of thing I have been describing doesn't go on very long. If it did the battery would run down inside itself and then when we came to start our automobile we would have to get out and crank.

How long does it go on? Answer another question first. So far we haven't connected any wire between the two plates of the battery, and so none of the electrons on the negative plate have any way of getting around to the positive plate where electrons are badly needed. Every time a negative sulphate ion combines with the spongy lead of the negative plate there are two more electrons added to that plate. You know how well electrons like each other. Do they let the sulphate ions keep giving that plate more electrons? There is the other question; and the answer is that they do not. Every electron that is added to that plate makes it just so much harder for another sulphate ion to get near enough to do business at all. That's why after a few extra electrons have acc.u.mulated on the spongy lead plate the actions which I was describing come to a stop.

Do they ever begin again? They do just as soon as there is any reduction in the number of electrons which are hopping around in the negative plate trying to keep out of each other's way. When we connect a wire between the plates we let some of these extra electrons of the negative plate pa.s.s along to the positive plate where they will be welcome. And the moment a couple of them start off on that errand along comes another sulphate ion in the solution and lands two more electrons on the plate.

That's how the battery keeps on discharging.

We mustn't let it get too much discharged for the lead sulphate is not soluble, as I just told you, and it will coat up that plate until there isn't much chance of getting the process to reverse. That's why we are so careful not to let the discharge process go on too long before we reverse it and charge. That's why, when the car battery has been used pretty hard to start the car, I like to run quite a while to let the generator charge the battery again. When the battery charges, the process reverses and we get spongy lead on the negative plate and lead peroxide on the positive plate.

You've learned enough for one day. Write me your questions and I'll answer and then go on in my next letter to tell how the audion works.

You know about conduction of electricity in wires; that is, about the electron stream, and about batteries which can cause the stream. Now you are ready for the most wonderful little device known to science: the audion.

LETTER 5

GETTING ELECTRONS FROM A HEATED WIRE

DEAR SON:

I was pleased to get your letter and its questions. Yes, a proton is a speck of electricity of the kind we call positive and an electron is of the kind we call negative. You might remember this simple law; "Like kinds of electricity repel, and unlike attract."

The word ion[2] is used to describe any atom, or part of a molecule which can travel by itself and has more or less than its proper number of electrons. By proper number of electrons I mean proper for the number of protons which it has. If an ion has more electrons than protons it is negative; if the inequality is the other way around it is positive. An atom or molecule has neither more nor less protons than electrons. It is neutral or "uncharged," as we say.

No, not every substance which will dissolve will dissociate or split up into positive and negative ions. The salt which you eat will, but the sugar will not. If you want a name for those substances which will dissociate in solution, call them "electrolytes." To make a battery we must always use an electrolyte.

Yes, it is hard to think of a smooth piece of metal or a wire as full of holes. Even in the densest solids like lead the atoms are quite far apart and there are large s.p.a.ces between the nuclei and the planetary electrons of each atom.

I hope this clears up the questions in your mind for I want to get along to the vacuum tube. By a vacuum we mean a s.p.a.ce which has very few atoms or molecules in it, just as few as we can possibly get, with the best methods of pumping and exhausting. For the present let's suppose that we can get all the gas molecules, that is, all the air, out of a little gla.s.s bulb.

The audion is a gla.s.s bulb like an electric light bulb which has in it a thread, or filament, of metal. The ends of this filament extend out through the gla.s.s so that we may connect a battery to them and pa.s.s a current of electricity through the wire. If we do so the wire gets hot.

What do we mean when we say "the wire gets hot?" We mean that it feels hot. It heats the gla.s.s bulb and we can feel it. But what do we mean in words of electrons and atoms? To answer this we must start back a little way.

In every bit of matter in our world the atoms and molecules are in very rapid motion. In gases they can move anywhere; and do. That's why odors travel so fast. In liquids most of the molecules or atoms have to do their moving without getting out of the dish or above the surface. Not all of them stay in, however, for some are always getting away from the liquid and going out into the air above. That is why a dish of water will dry up so quickly. The faster the molecules are going the better chance they have of jumping clear away from the water like fish jumping in the lake at sundown. Heating the liquid makes its molecules move faster and so more of them are able to jump clear of the rest of the liquid. That's why when we come in wet we hang our clothes where they will get warm. The water in them evaporates more quickly when it is heated because all we mean by "heating" is speeding up the molecules.

In a solid body the molecules can't get very far away from where they start but they keep moving back and forth and around and around. The hotter the body is, the faster are its molecules moving. Generally they move a little farther when the body is hot than when it is cold. That means they must have a little more room and that is why a body is larger when hot than when cold. It expands with heating because its molecules are moving more rapidly and slightly farther.

When a wire is heated its molecules and atoms are hurried up and they dash back and forth faster than before. Now you know that a wire, like the filament of a lamp, gets hot when the "electricity is turned on,"

that is, when there is a stream of electrons pa.s.sing through it. Why does it get hot? Because when the electrons stream through it they b.u.mp and jostle their way along like rude boys on a crowded sidewalk. The atoms have to step a bit more lively to keep out of the way. These more rapid motions of the atoms we recognize by the wire growing hotter.

That is why an electric current heats a wire through which it is flowing. Now what happens to the electrons, the rude boys who are dodging their way along the sidewalk? Some of them are going so fast and so carelessly that they will have to dodge out into the gutter and off the sidewalk entirely. The more boys that are rus.h.i.+ng along and the faster they are going the more of them will be turned aside and plunge off the sidewalks.

The greater and faster the stream of electrons, that is the more current which is flowing through the wire, the more electrons will be "emitted,"

that is, thrown out of the wire. If you could watch them you would see them shooting out of the wire, here, there, and all along its length, and going in every direction. The number which shoot out each second isn't very large until they have stirred things up so that the wire is just about red hot.

What becomes of them? Sometimes they don't get very far away from the wire and so come back inside again. They scoot off the sidewalk and on again just as boys do in dodging their way along. Some of them start away as if they were going for good.

If the wire is in a vacuum tube, as it is in the case of the audion, they can't get very far away. Of course there is lots of room; but they are going so fast that they need more room just as older boys who run fast need a larger play ground than do the little tots. By and by there gets to be so many of them outside that they have to dodge each other and some of them are always dodging back into the wire while new electrons are shooting out from it.

When there are just as many electrons dodging back into the wire each second as are being emitted from it the vacuum in the tube has all the electrons it can hold. We might say it is "saturated" with electrons, which means, in slang, "full up." If any more electrons are to get out of the filament just as many others which are already outside have to go back inside. Or else they have got to be taken away somewhere else.

What I have just told you about electrons getting away from a heated wire is very much like what happens when a liquid is heated. The molecules of the liquid get away from the surface. If we cover a dish of liquid which is being heated the liquid molecules can't get far away and very soon the s.p.a.ce between the surface of the liquid and the cover gets saturated with them. Then every time another molecule escapes from the surface of the liquid there must be some molecule which goes back into the liquid. There is then just as much condensation back into liquid as there is evaporation from it. That's why in cooking they put covers over the vessels when they don't want the liquid all to "boil away."