Letters of a Radio-Engineer to His Son Part 1

Letters of a Radio-Engineer to His Son.

by John Mills.

LETTER 1

ELECTRICITY AND MATTER

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.

All the letters will be written just as I would talk to you, for I shall draw little sketches as I go along. One of them will tell you how to experiment for yourself. This will be the most interesting of all. You can find plenty of books to tell you how radio sets operate and what to do, but very few except some for advanced students tell you how to experiment for yourself. Not to waste time in your own experiments, however, you will need to be quite familiar with the ideas of the other letters.

What is a radio set? Copper wires, tinfoil, gla.s.s 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.

How else do they differ? They differ in their actions. Protons don't like to a.s.sociate with other protons but take quite keenly to electrons.

And electrons--they go with protons but they won't a.s.sociate with each other. An electron always likes to be close to a proton. Two is company when one is an electron and the other a proton but three is a crowd always.

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 s.p.a.ce so that lots of other electrons or protons could be fitted in between if only they were willing to get that close together.

Sometimes they do get very close together. I can tell you how if you will imagine four small boys playing tag. Suppose Tom and d.i.c.k don't like to play with each other and run away from each other if they can.

Now suppose that Bill and Sam won't play with each other if they can help it but that either of them will play with Tom or d.i.c.k whenever there is a chance. Now suppose Tom and Bill see each other; they start running toward each other to get up some sort of a game. But Sam sees Tom at the same time, so he starts running to join him even though Bill is going to be there too. Meanwhile d.i.c.k sees Bill and Sam running along and since they are his natural playmates he follows them. In a minute they are all together, and playing a great game; although some of the boys don't like to play 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 sort of a molecule is formed will depend upon how many and what kinds of atoms group together to play the larger game. Whenever there is a big game it doesn't mean that the little atomic groups which enter into it are all changed around. They keep together like a troop of boy scouts in a grand picnic in which lots of troops are present. At any rate they keep together enough so that we can still call them a group, that is an atom, even though they do adapt their game somewhat so as to fit in with other groups--that is with other atoms.

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 a.s.sociating 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.

That hydrogen atom is responsible for lots of interesting things for it is a great one to join with other atoms. We don't often find it by itself although we can make it change its partners and go from one molecule to another very easily. That is what happens every time you stain anything with acid. A hydrogen atom leaves a molecule of the acid and then it isn't acid any more. What remains isn't a happy group either for it has lost some of its playfellows. The hydrogen goes and joins with the stuff which gets stained. But it doesn't join with the whole molecule; it picks out part of it to a.s.sociate with and that leaves the other part to take the place of the hydrogen in the original molecule of acid from which it came. Many of the actions which we call chemistry are merely the result of such changes of atoms from one molecule to another.

Not only does the hydrogen atom like to a.s.sociate 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."

LETTER 2

WHY A COPPER WIRE WILL CONDUCT ELECTRICITY

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 quant.i.ties 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 airs.h.i.+ps, because it cannot burn or explode. It is not as light as hydrogen but it serves quite well for making balloons buoyant in air.

This helium atom is made up of four electrons and four protons. Right at the center there is a small closely crowded group which contains all the protons and two of the electrons. The other two electrons play around quite a little way from this inner group. It will make our explanations easier if we learn to call this inner group "the nucleus" of the atom.

It is the center of the atom and the other two electrons play around about it just as the earth and Mars and the other planets play or revolve about the sun as a center. That is why we shall call these two electrons "planetary electrons."

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.

[Ill.u.s.tration: Pl. II.--Bird's-eye View of Radio Central (Courtesy of Radio Corporation of America).]

The earth and rocks also contain a great deal of oxygen; about 47.3 percent of the atoms which form earth and rocks are oxygen atoms. About half of the rest of the atoms are of a kind called silicon. Sand is made up of atoms of silicon and oxygen and you know how much sand there is.

About 27.7 percent of the earth and its rocks is silicon. The next most important kind of atom in the earth is aluminum and after that iron and then calcium. Here is the way they run in percentages: Aluminum 7.8 percent; iron 4.5 percent; calcium 3.5 percent; sodium 2.4 percent; pota.s.sium 2.4 percent; magnesium 2.2 percent. Besides these which are most important there is about 0.2 percent of hydrogen and the same amount of carbon. Then there is a little phosphorus, a little sulphur, a little fluorine, and small amounts of all of the rest of the different kinds of atoms.

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.

Most of sea water, therefore, is just water, that is, pure water. But it contains some other substances as well and the best known of these is salt. Salt is a substance the molecules of which contain atoms of sodium and of chlorine. That is why sea water is about 1.1 percent sodium and about 2.1 percent chlorine. There are some other kinds of atoms in sea water, as you would expect, for it gets all the substances which the waters of the earth dissolve and carry down to it but they are unimportant in amounts.

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.

We might use numbers for the different kinds of atoms instead of names if we wanted to do so. We could describe any kind of atom by telling how many planetary electrons there were in it. For example, hydrogen would be number 1, helium number 2, lithium of which you perhaps never heard, would be number 3, and so on. Oxygen is 8, sodium is 11, chlorine is 17, iron 26, and copper 29. For each kind of atom there is a number. Let's call that number its atomic number.

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.

About the nucleus of a copper atom there are playing 29 electrons just as if the nucleus was a teacher responsible for 29 children who were out in the play yard. There is one very funny thing about it all, however, and that is that we must think of the scholars as if they were all just alike so that the teacher couldn't tell one from the other. Electrons are all alike, you remember. All the teacher or nucleus cares for is that there shall be just the right number playing around her. You could bring a boy in from some other play ground and the teacher couldn't tell that he was a stranger but she would know that something was the matter for there would be one too many in her group. She is responsible for just 29 scholars, and the nucleus of the copper atom is responsible for just 29 electrons. It doesn't make any difference where these electrons come from provided there are always just 29 playing around the nucleus.

If there are more or less than 29 something peculiar will happen.

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.

We don't know all we might about how the electrons wander around from atom to atom inside a copper wire but we do know that there are always a lot of them moving about in the s.p.a.ces between the atoms. Some of them are going one way and some another.

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.