Part 4 (1/2)

In most of the electrical circuits hich you will deal you will find that electronsrate if there is to be any appreciable effect When you turn on the 40-watt light at your desk you start theh 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 sidehich the filao by We don't try to count theo 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 hi in a e 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 strea a current of one ah 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 auto you would take a scale and weigh it, wouldn't you? You s out in the kitchen But if the spring balance said the thing weighed five pounds hoould you know if it was right? Of course you h it on soave correct weight?

The only way to find out would be to try the scales eights which you were sure were right and see if the readings on the scale correspond to the knoeights Then you could trust it to tell you the weight of so else That's the way scales are tested In fact that's the way that the makers kno to hts and ures 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 hts, that is, by calibration

Hoould you know that the weights you used to calibrate your scale were really what you thought them to be? You would have to find soree was correct and then coht, for exaht to the Bureau of Standards in Washi+ngton 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 platinuht 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 isin it to h the instrument We want a meter for the flow of electrons In the baseas and another for the floater Each of these has soas 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 odoe” or the ”total e” We want a meter like the speedometer which will indicate at each instant just how fast the electrons are streah 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 ant to know the electron streah 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 buh 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 firer the rate of flow of electrons the hotter it gets; and the hotter it gets the ht fasten one end--the short end--of a little lever A se 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 nifies the motion of the center of the hot wire part of our meter so that we can see it easier

[Illustration: Fig 10]

There are several ways to10 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 a we could put a numeral ”1” opposite where the pointer stood Then if we could increase the current until there were two ah the meter we could mark that position of the pointer ”2” and so on That's the ould calibrate the meter

After we had done so ould call it an ”ao people would have called it an ”amperemeter” but no one who is up-to-date would call it so to-day

[Illustration: Fig 11]

If we had a very carefully made ammeter ould send it to the Bureau of Standards to be calibrated At the Bureau they have a nus They would put one of their meters and ours into the same circuit so that both carry the sa 11 Then whatever the reading was on their meter could be marked opposite the pointer on ours

Noant to tell you how the physicists at the Bureau knohat is an aress of physicists and electrical engineers froht should be the unit in which to measure current They decided just what they would call an ampere and then all the countries fro that an ampere should be what these scientists had recommended To-day, therefore, an ampere is defined by law

To tell when an a requires the use of two silver plates and a solution of silver nitrate Silver nitrate has roup of atoms called ”nitrate” You remember that the molecule of copper sulphate, discussed in our third letter, was forroup called sulphate Nitrate is another group soen atoms in it, but it has three instead of four, and instead of a sulphur atoen

When silver nitrate o 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 foren 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 froives electrons to the other So of electrons are just like the silver ions which are o 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 soet out of the solution They aren't going back to the positive silver plate froative plate where the battery is sending an electron for every one which it takes away froative 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 froot this electron it is again a normal silver atom So it stays with the other ato For every atom which is lost froative 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 froative plate there is added to the negative plate an atohed 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 sah the circuit each second Then if we kno long the current has been running we can tell how much silver has been deposited each second