LETTERS OF A RADIO-ENGINEER TO HIS SON

My Dear Son:

You are interested in radio-telephony and want me to explain it to you. I’ll do so in the shortest and easiest way which I can devise. The explanation will be the simplest which I can give and still make it possible for you to build and operate your own set and to understand the operation of the large commercial sets to which you will listen.

I’ll write you a series of letters which will contain only what is important in the radio of to-day and those ideas which seem necessary if you are to follow the rapid advances which radio is making. Some of the letters you will find to require a second reading and study. In the case of a few you might postpone a second reading until you have finished those which interest you most. I’ll mark the letters to omit in this way.

What is a radio set? Copper wires, tinfoil, glass plates, sheets of mica, metal, and wood. Where does it get its ability to work–that is, where does the “energy” come from which runs the set? From batteries or from dynamos. That much you know already, but what is the real reason that we can use copper wires, metal plates, audions, crystals, and batteries to send messages and to receive them?

The reason is that all these things are made of little specks, too tiny ever to see, which we might call specks of electricity. There are only two kinds of specks and we had better give them their right names at once to save time. One kind of speck is called “electron” and the other kind “proton.” How do they differ? They probably differ in size but we don’t yet know so very much about their sizes. They differ in laziness a great deal. One is about 1845 times as lazy as the other. That is, it has eighteen hundred and forty-five times as much inertia as the other. It is harder to get it started but it is just as much harder to get it to stop after it is once started or to change its direction and go a different direction. The proton has the larger inertia. It is the electron which is the easier to start or stop.

It doesn’t make any difference to a proton what electron it is keeping company with provided only it is an electron and not another proton. All electrons are alike as far as we can tell and so are all protons. That means that all the stuff, or matter, of our world is made up of two kinds of building blocks, and all the blocks of each kind are just alike. Of course you mustn’t think of these blocks as like bricks, for we don’t know their shapes.

Then there is another reason why you must not think of them as bricks and that is because when you build a house out of bricks each brick must rest on another. Between an electron and any other electron or between two protons or between an electron and a proton there is usually a relatively enormous distance. There is enough space so that lots of other electrons or protons could be fitted in between if only they were willing to get that close together.

Whenever there is a group of protons and electrons playing together we have what we call an “atom.” There are about ninety different games which electrons and protons can play, that is ninety different kinds of atoms. These games differ in the number of electrons and protons who play and in the way they arrange themselves. Larger games can be formed if a number of atoms join together. Then there is a “molecule.” Of molecules there are as many kinds as there are different substances in the world. It takes a lot of molecules together to form something big enough to see, for even the largest molecule, that of starch, is much too small to be seen by itself with the best possible microscope.

What will the kind of atom depend upon? It will depend upon how many electrons and protons are grouped together in it to play their little game. How any atom behaves so far as associating with other groups or atoms will depend upon what sort of a game its own electrons and protons are playing.

Now the simplest kind of a game that can be played, and the one with the smallest number of electrons and protons, is that played by a single proton and a single electron. I don’t know just how it is played but I should guess that they sort of chase each other around in circles. At any rate I do know that the atom called “hydrogen” is formed by just one proton and one electron. Suppose they were magnified until they were as large as the moon and the earth. Then they would be just about as far apart but the smaller one would be the proton.

Not only does the hydrogen atom like to associate in a larger game with other kinds of atoms but it likes to do so with one of its own kind. When it does we have a molecule of hydrogen gas, the same gas as is used in balloons.

We haven’t seemed to get very far yet toward radio but you can see how we shall when I tell you that next time I shall write of more complicated games such as are played in the atoms of copper which form the wires of radio sets and of how these wires can do what we call “carrying an electric current.”

My Dear Young Atomist:

You have learned that the simplest group which can be formed by protons and electrons is one proton and one electron chasing each other around in a fast game. This group is called an atom of hydrogen. A molecule of hydrogen is two of these groups together.

All the other possible kinds of groups are more complicated. The next simplest is that of the atom of helium. Helium is a gas of which small quantities are obtained from certain oil wells and there isn’t very much of it to be obtained. It is an inert gas, as we call it, because it won’t burn or combine with anything else. It doesn’t care to enter into the larger games of molecular groups. It is satisfied to be as it is, so that it isn’t much use in chemistry because you can’t make anything else out of it. That’s the reason why it is so highly recommended for filling balloons or airships, because it cannot burn or explode. It is not as light as hydrogen but it serves quite well for making balloons buoyant in air.

There are about ninety different kinds of atoms and they all have names. Some of them are more familiar than hydrogen and helium. For example, there is the iron atom, the copper atom, the sulphur atom and so on. Some of these atoms you ought to know and so, before telling you more of how atoms are formed by protons and electrons, I am going to write down the names of some of the atoms which we have in the earth and rocks of our world, in the water of the oceans, and in the air above.

Start first with air. It is a mixture of several kinds of gases. Each gas is a different kind of atom. There is just a slight trace of hydrogen and a very small amount of helium and of some other gases which I won’t bother you with learning. Most of the air, however, is nitrogen, about 78 percent in fact and almost all the rest is oxygen. About 20.8 percent is oxygen so that all the gases other than these two make up only about 1.2 percent of the atmosphere in which we live.

Sea water is mostly oxygen and hydrogen, about 85.8 percent of oxygen and 10.7 percent of hydrogen. That is what you would expect for water is made up of molecules which in turn are formed by two atoms of hydrogen and one atom of oxygen. The oxygen atom is about sixteen times as heavy as the hydrogen atom. However, for every oxygen atom there are two hydrogen atoms so that for every pound of hydrogen in water there are about eight pounds of oxygen. That is why there is about eight times as high a percentage of oxygen in sea water as there is of hydrogen.

Now we know something about the names of the important kinds of atoms and can take up again the question of how they are formed by protons and electrons. No matter what kind of atom we are dealing with we always have a nucleus or center and some electrons playing around that nucleus like tiny planets. The only differences between one kind of atom and any other kind are differences in the nucleus and differences in the number and arrangement of the planetary electrons which are playing about the nucleus.

No matter what kind of atom we are considering there is always in it just as many electrons as protons. For example, the iron atom is formed by a nucleus and twenty-six electrons playing around it. The copper atom has twenty-nine electrons as tiny planets to its nucleus. What does that mean about its nucleus? That there are twenty-nine more protons in the nucleus than there are electrons. Silver has even more planetary electrons, for it has 47. Radium has 88 and the heaviest atom of all, that of uranium, has 92.

Now let’s see what the atomic number tells us. Take copper, for example, which is number 29. In each atom of copper there are 29 electrons playing around the nucleus. The nucleus itself is a little inner group of electrons and protons, but there are more protons than electrons in it; twenty-nine more in fact. In an atom there is always an extra proton in the nucleus for each planetary electron. That makes the total number of protons and electrons the same.

We shall see later what might happen, but first let’s think of an enormous lot of atoms such as there would be in a copper wire. A small copper wire will have in it billions of copper atoms, each with its planetary electrons playing their invisible game about their own nucleus. There is quite a little distance in any atom between the nucleus and any of the electrons for which it is responsible. There is usually a greater distance still between one atomic group and any other.

On the whole the electrons hold pretty close to their own circles about their own nuclei. There is always some tendency to run away and play in some other group. With 29 electrons it’s no wonder if sometimes one goes wandering off and finally gets into the game about some other nucleus. Of course, an electron from some other atom may come wandering along and take the place just left vacant, so that nucleus is satisfied.

It’s these wandering electrons which are affected when a battery is connected to a copper wire. Every single electron which is away from its home group, and wandering around, is sent scampering along toward the end of the wire which is connected to the positive plate or terminal of the battery and away from the negative plate. That’s what the battery does to them for being away from home; it drives them along the wire. There’s a regular stream or procession of them from the negative end of the wire toward the positive. When we have a stream of electrons like this we say we have a current of electricity.

My Dear Boy:

When I was a boy we used to make our own batteries for our experiments. That was before storage batteries became as widely used as they are to-day when everybody has one in the starting system of his automobile. That was also before the day of the small dry battery such as we use in pocket flash lights. The batteries which we made were like those which they used on telegraph systems, and were sometimes called “gravity” batteries. Of course, we tried several kinds and I believe I got quite a little acid around the house at one time or another. I’ll tell you about only one kind but I shall use the words “electron,” “proton,” “nucleus,” “atom,” and “molecule,” about some of which nothing was known when I was a boy.

We used a straight-sided glass jar which would hold about a gallon. On the bottom we set a star shaped arrangement made of sheets of copper with a long wire soldered to it so as to reach up out of the jar. Then we poured in a solution of copper sulphate until the jar was about half full. This solution was made by dissolving in water crystals of “blue vitriol” which we bought at the drug store.

When it dissolves in water the molecules of the blue vitriol go wandering out into the spaces between the water molecules. But that isn’t all that happens or the most important thing for one who is interested in making a battery.

Each molecule is formed by six atoms, that is by six little groups of electrons playing about six little nuclei. About each nucleus there is going on a game but some of the electrons are playing in the game about their own nucleus and at the same time taking some part in the game which is going on around one of the other nuclei. That’s why the groups or atoms stay together as a molecule. When the molecules wander out into the spaces between the water molecules something happens to this complicated game.

It will be easiest to see what sort of thing happens if we talk about a molecule of ordinary table salt, for that has only two atoms in it. One atom is sodium and one is chlorine. The sodium molecule has eleven electrons playing around its nucleus. Fairly close to the nucleus there are two electrons. Then farther away there are eight more and these are having a perfect game. Then still farther away from the nucleus there is a single lonely electron.

I am going to draw a picture (Fig. 1) to show what I mean, but you must remember that these electrons are not all in the same plane as if they lay on a sheet of paper, but are scattered all around just as they would be if they were specks on a ball.

You see that the sodium atom has one lonely electron which hasn’t any play fellows and that the chlorine atom has seven in its outside circle. It appears that eight would make a much better game. Suppose that extra electron in the sodium atom goes over and plays with those in the chlorine atom so as to make eight in the outside group as I have shown Fig. 2. That will be all right as long as it doesn’t get out of sight of its own nucleus because you remember that the sodium nucleus is responsible for eleven electrons. The lonely electron of the sodium atom needn’t be lonely any more if it can persuade its nucleus to stay so close to the chlorine atom that it can play in the outer circle of the chlorine atom.

This molecule which is formed by a sodium atom and a chlorine atom is called a molecule of sodium chloride by chemists and a molecule of salt by most every one who eats it. Something strange happens when it dissolves. It wanders around between the water molecules and for some reason or other–we don’t know exactly why–it decides to split up again into sodium and chlorine but it can’t quite do it. The electron which joined the game about the chlorine nucleus won’t leave it. The result is that the nucleus of the sodium atom gets away but it leaves this one electron behind.

You remember that in an atom there are always just as many protons as electrons. In this sodium ion which is formed when the nucleus of the sodium atom breaks away but leaves behind one planetary electron, there is then one more proton than there are electrons. Because it has an extra proton, which hasn’t any electron to associate with, we call it a plus ion or a “positive ion.” Similarly we call the chlorine ion, which has one less proton than it has electrons, a minus or “negative ion.”

Now, despite the fact that these ions broke away from each other they aren’t really satisfied. Any time that the sodium ion can find an electron to take the place of the one it lost it will welcome it. That is, the sodium ion will want to go toward places where there are extra electrons. In the same way the chlorine ion will go toward places where electrons are wanted as if it could satisfy its guilty conscience by giving up the electron which it stole from the sodium atom, or at least by giving away some other electron, for they are all alike anyway.

Now we can see what happens when copper sulphate dissolves. The copper atom has twenty-nine electrons about its nucleus and all except two of these are nicely grouped for playing their games about the nucleus. Two of the electrons are rather out of the game, and are unsatisfied. They play with the electrons of the part of the molecule which is called “sulphate,” that is, the part formed by the sulphur atom and the four oxygen atoms. These five atoms of the sulphate part stay together very well and so we treat them as a group.

The sulphate group and the copper atom stay together as long as they are not in solution but when they are, they act very much like the sodium and chlorine which I just described. The molecule splits up into two ions, one positive and one negative. The positive ion is the copper part except that two of the electrons which really belong to a copper atom got left behind because the sulphate part wouldn’t give them up. The rest of the molecule is the negative ion.

Just like a sodium ion the copper ion will tend to go toward any place where there are extra electrons which it can get to satisfy its own needs. In much the same way the sulphate ion will go toward places where it can give up its two extra electrons. Sometimes, of course, as ions of these two kinds wander about between the water molecules, they meet and satisfy each other by forming a molecule of copper sulphate. But if they do they will split apart later on; that is, they will “dissociate” as we should say.

Now let’s go on with the kind of batteries I used to make as a boy. You can see that in the solution of copper sulphate at the bottom of the jar there was always present a lot of positive copper ions and of negative sulphate ions.

I remember when I mixed the sulphuric acid with water that I learned to pour the acid into the water and not the other way around. Spatterings of sulphuric acid are not good for hands or clothes. With this solution I filled the jar almost to the top and then hung over the edge a sort of a crow’s foot shape of cast zinc. The zinc reached down into the sulphuric acid solution. There was a binding post on it to which a wire could be connected. This wire and the one which came from the plate of copper at the bottom were the two terminals of the battery. We called the wire from the copper “positive” and the one from the zinc “negative.”

Now we shall see why and how the battery worked. The molecules of sulphuric acid dissociate in solution just as do those of copper sulphate. When sulphuric acid molecules split, the sulphate part goes away with two electrons which don’t belong to it and each of the hydrogen atoms goes away by itself but without its electron. We call each a “hydrogen ion” but you can see that each is a single proton.

What happens then is this. The sulphate ions wandering around in the weak solution of sulphuric acid come along beside the zinc plate and beckon to its atoms. The sulphate ions had a great deal rather play the game called “zinc sulphate” than the game called “hydrogen sulphate.” So the zinc atoms leave their places to join with the sulphate ions. But wait a minute! The sulphate ions have two extra electrons which they kept from the hydrogen atoms. They don’t need the two lonely electrons which each zinc atom could bring and so the zinc atom leaves behind it these unnecessary electrons.

Every time a zinc atom leaves the plate it fails to take all its electrons with it. What leaves the zinc plate, therefore, to go into solution is really not a zinc atom but is a zinc ion; that is, it is the nucleus of a zinc atom and all except two of the planetary electrons.

Every time a zinc ion leaves the plate there are left behind two electrons. The plate doesn’t want them for all the rest of its atoms have just the same number of protons as of electrons. Where are they to go? We shall see in a minute.

Before the zinc ions began to crowd in there were just enough hydrogen ions to go with the sulphate ions. As it is, the entrance of the zinc ions has increased the number of positive ions and now there are too many. Some of the positive ions, therefore, and particularly the hydrogen ions, because the sulphate prefers to associate with the zinc ions, can’t find enough playfellows and so go down in the jar.

Down in the bottom of the jar the hydrogen ions find more sulphate ions to play with, but that leaves the copper ions which used to play with these sulphate ions without any playmates. So the copper ions go still further down and join with the copper atoms of the copper plate. They haven’t much right to do so, for you remember that they haven’t their proper number of electrons. Each copper ion lacks two electrons of being a copper atom. Nevertheless they join the copper plate. The result is a plate of copper which has too few electrons. It needs two electrons for every copper ion which joins it.

That’s the sort of a battery I used to play with. If you understand it you can get the general idea of all batteries. Let me express it in general terms.

At the negative plate of a battery ions go into solution and electrons are left behind. At the other end of the battery positive ions are crowded out of solution and join the plate where they cause a scarcity of electrons; that is, make the plate positive. If a wire is connected between the two plates, electrons will stream through it from the negative plate to the positive; and this stream is a current of electricity.