Chapter VIII: Letter 7: How to Measure an Electron Stream

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

DEAR YOUTH:

If we are to talk about the audion and how its grid controls the current in the plate circuit we must know something of how to measure currents. An electric current is a stream of electrons. We measure it by finding the rate at which electrons are traveling along through the circuit.

What do we mean by the word "rate?" You know what it means when a speedometer says twenty miles an hour. If the car should keep going just as it was doing at the instant you looked at the speedometer it would go twenty miles in the next hour. Its rate is twenty miles an hour even though it runs into a smash the next minute and never goes anywhere again except to the junk heap.

It's the same when we talk of electric currents. We say there is a current of such and such a number of electrons a second going by each point in the circuit. We don't mean that the current isn't going to change, for it may get larger or smaller, but we do mean that if the stream of electrons keeps going just as it is there will be such and such a number of electrons pass by in the next second.

In most of the electrical circuits with which you will deal you will find that electrons must be passing along in the circuit at a most amazing rate if there is to be any appreciable effect. When you turn on the 40-watt light at your desk you start them going through the filament of the lamp at the rate of about two and a half billion billion each second. You have stood on the sidewalk in the city and watched the people stream past you. Just suppose you could stand beside that narrow little sidewalk which the filament offers to the electrons and count them as they go by. We don't try to count them although we do to-day know about how many go by in a second if the current is steady.

If some one asks you how old you are you don't say "About five hundred million seconds"; you tell him in years. When some one asks how large a current is flowing in a wire we don't tell him six billion billion electrons each second; we tell him "one ampere." Just as we use years as the units in which to count up time so we use amperes as the units in which to count up streams of electrons. When a wire is carrying a current of one ampere the electrons are streaming through it at the rate of about 6,000,000,000,000,000,000 a second.

Don't try to remember this number but do remember that an ampere is a unit in which we measure currents just as a year is a unit in which we measure time. An ampere is a unit in which we measure streams of electrons just as "miles per hour" is a unit in which we measure the speed of trains or automobiles.

If you wanted to find the weight of something you would take a scale and weigh it, wouldn't you? You might take that spring balance which hangs out in the kitchen. But if the spring balance said the thing weighed five pounds how would you know if it was right? Of course you might take what ever it was down town and weigh it on some other scales but how would you know those scales gave correct weight?

The only way to find out would be to try the scales with weights which you were sure were right and see if the readings on the scale correspond to the known weights. Then you could trust it to tell you the weight of something else. That's the way scales are tested. In fact that's the way that the makers know how to mark them in the first place. They put on known weights and marked the lines and figures which you see. What they did was called "calibrating" the scale. You could make a scale for yourself if you wished, but if it was to be reliable you would have to find the places for the markings by applying known weights, that is, by calibration.

How would you know that the weights you used to calibrate your scale were really what you thought them to be? You would have to find some place where they had a weight that everybody would agree was correct and then compare your weight with that. You might, for example, send your pound weight to the Bureau of Standards in Washington and for a small payment have the Bureau compare it with the pound which it keeps as a standard.

That is easy where one is interested in a pound. But it is a little different when one is interested in an ampere. You can't make an ampere out of a piece of platinum as you can a standard pound weight. An ampere is a stream of electrons at about the rate of six billion billion a second. No one could ever count anywhere near that many, and yet everybody who is concerned with electricity wants to be able to measure currents in amperes. How is it done?

First there is made an instrument which will have something in it to move when electrons are flowing through the instrument. We want a meter for the flow of electrons. In the basement we have a meter for the flow of gas and another for the flow of water. Each of these has some part which will move when the water or the gas passes through. But they are both arranged with little gear wheels so as to keep track of all the water or gas which has flowed through; they won't tell the rate at which the gas or water is flowing. They are like the odometer on the car which gives the "trip mileage" or the "total mileage." We want a meter like the speedometer which will indicate at each instant just how fast the electrons are streaming through it.

There are several kinds of meters but I shall not try to tell you now of more than one. The simplest to understand is called a "hot-wire meter." You already know that an electron stream heats a wire. Suppose a piece of fine wire is fastened at the two ends and that there are binding posts also fastened to these ends of the wire so that the wire may be made part of the circuit where we want to know the electron stream. Then the same stream of electrons will flow through the fine wire as through the other parts of the circuit. Because the wire is fine it acts like a very narrow sidewalk for the stream of electrons and they have to bump and jostle pretty hard to get through. That's why the wire gets heated.

You know that a heated wire expands. This wire expands. It grows longer and because it is held firmly at the ends it must bow out at the center. The bigger the rate of flow of electrons the hotter it gets; and the hotter it gets the more it bows out. At the center we might fasten one end--the short end--of a little lever. A small motion of this short end of the lever will mean a large motion of the other end, just like a "teeter board" when one end is longer than the other; the child on the long end travels further than the child on the short end. The lever magnifies the motion of the center of the hot wire part of our meter so that we can see it easier.

There are several ways to make such a meter. The one shown in Fig. 10 is as easy to understand as any. We shape the long end of the lever like a pointer. Then the hotter the wire the farther the pointer moves.

If we could put this meter in an electric circuit where we knew one ampere was flowing we could put a numeral "1" opposite where the pointer stood. Then if we could increase the current until there were two amperes flowing through the meter we could mark that position of the pointer "2" and so on. That's the way we would calibrate the meter. After we had done so we would call it an "ammeter" because it measures amperes. Years ago people would have called it an "amperemeter" but no one who is up-to-date would call it so to-day.

If we had a very carefully made ammeter we would send it to the Bureau of Standards to be calibrated. At the Bureau they have a number of meters which they know are correct in their readings. They would put one of their meters and ours into the same circuit so that both carry the same stream of electrons as in Fig. 11. Then whatever the reading was on their meter could be marked opposite the pointer on ours.

Now I want to tell you how the physicists at the Bureau know what is an ampere. Several years ago there was a meeting or congress of physicists and electrical engineers from all over the world who discussed what they thought should be the unit in which to measure current. They decided just what they would call an ampere and then all the countries from which they came passed laws saying that an ampere should be what these scientists had recommended. To-day, therefore, an ampere is defined by law.

To tell when an ampere of current is flowing requires the use of two silver plates and a solution of silver nitrate. Silver nitrate has molecules made up of one atom of silver combined with a group of atoms called "nitrate." You remember that the molecule of copper sulphate, discussed in our third letter, was formed by a copper atom and a group called sulphate. Nitrate is another group something like sulphate for it has oxygen atoms in it, but it has three instead of four, and instead of a sulphur atom there is an atom of nitrogen.

When silver nitrate molecules go into solution they break up into ions just as copper sulphate does. One ion is a silver atom which has lost one electron. This electron was stolen from it by the nitrate part of the molecule when they dissociated. The nitrate ion, therefore, is formed by a nitrogen atom, three oxygen atoms, and one extra electron.

If we put two plates of silver into such a solution nothing will happen until we connect a battery to the plates. Then the battery takes electrons away from one plate and gives electrons to the other. Some of the atoms in the plate which the battery is robbing of electrons are just like the silver ions which are moving around in the solution. That's why they can go out into the solution and play with the nitrate ions each of which has an extra electron which it stole from some silver atom. But the moment silver ions leave their plate we have more silver ions in the solution than we do sulphate ions.

The only thing that can happen is for some of the silver ions to get out of the solution. They aren't going back to the positive silver plate from which they just came. They go on toward the negative plate where the battery is sending an electron for every one which it takes away from the positive plate. There start off towards the negative plate, not only the ions which just came from the positive plate, but all the ions that are in the solution. The first one to arrive gets an electron but it can't take it away from the silver plate. And why should it? As soon as it has got this electron it is again a normal silver atom. So it stays with the other atoms in the silver plate. That's what happens right along. For every atom which is lost from the positive plate there is one added to the negative plate. The silver of the positive plate gradually wastes away and the negative plate gradually gets an extra coating of silver.

Every time the battery takes an electron away from the positive plate and gives it to the negative plate there is added to the negative plate an atom of silver. If the negative plate is weighed before the battery is connected and again after the battery is disconnected we can tell how much silver has been added to it. Suppose the current has been perfectly steady, that is, the same number of electrons streaming through the circuit each second. Then if we know how long the current has been running we can tell how much silver has been deposited each second.

The law says that if silver is being deposited at the rate of 0.001118 gram each second then the current is one ampere. That's a small amount of silver, only about a thousandth part of a gram, and you know that it takes 28.35 grams to make an ounce. It's a very small amount of silver but it's an enormous number of atoms. How many? Six billion billion, of course, for there is deposited one atom for each electron in the stream.

In my next letter I'll tell you how we measure the pull which batteries can give to electrons, and then we shall be ready to go on with more about the audion.