Letters of a Radio-Engineer to His Son

by John Mills

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.

52That 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 53to 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 54where 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.