Part 4 (2/2)

The law says that if silver is being deposited at the rate of 0001118 gram each second then the current is one ampere That's a srarams 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 hoe ive to electrons, and then we shall be ready to go on with more about the audion

LETTER 8

ELECTRON-MOVING-FORCES

(This letter )

DEAR YOUNG MAN:

I trust you have a fairly good idea that an ampere means a stream of electrons at a certain definite rate and hence that a current of say 3 amperesalong each second

In the third and fourth letters you found out why a battery drives electrons around a conducting circuit You also found that there are several different kinds of batteries Batteries differ in their abilities to drive electrons and it is therefore convenient to have so the electron--force or ”electromotive force” which each battery can exert To express electroive the results of our measurements we must have some unit The unit we use is called the ”volt”

The volt is defined by law and is based on the suggestions of the same body of scientists who recommended the a how tothat this battery had an electromotive force of a certain number of volts One can buy such standard batteries, or standard cells as they are called, or he can ht he can then send them to the Bureau of Standards and have them compared with the standard cells which the Bureau has

I don't propose to tell you much about standard cells for you won't have to use them until you coood for ordinary purposes because theelectrons the conditions inside theed They are delicate little affairs and are useful only as standards hich to compare other batteries And even as standard batteries they must be used in such a way that they are not required to drive any electrons

[Illustration: Fig 12]

Let's see how it can be done Suppose two boys sit opposite each other on the floor and brace their feet together Then with their hands they take hold of a stick and pull in opposite directions If both have the same stick-motive-force the stick will not ative feet--I12 Then we connect their positive terether by a wire In the wire there will be lots of free electrons ready to go to the positive plate of the battery which pulls the harder If the batteries are equal in electroht where they are There will be no strea one of the batteries to coht, you think, but what are we to do when the batteries are not just equal in e m f? (e m f is code for electro includes so

[Illustration: Fig 13]

Suppose we take batteries which aren't going to be injured by being e batteries will do nicely--and connect the 13 When batteries are in series they act like a single stronger battery, one whose e m f is the sum of the e m f's of the separate batteries Connect these batteries to a long fine wire as in Fig 14

There is a streaative terative terainst each other Then connect a wire to the positive ter ar out fro 14]

Touch the end of the wire, which is _p_ of Fig 14, to soht at _a_, of course, there are some free electrons and they hear the calls of both batteries If the standard battery, _S_ of the figure, calls the stronger they go to it In that case move the end _p_ nearer the positive plate of the battery _B_, so that it will have a chance to exert a stronger pull Suppose we try at _c_ and find the battery _B_ is there the stronger Then we can move back to some point, say _b_, where the pulls are equal

Toinstrument in the hich leads from the positive ter fine wire so that there can never be much of an electron stream anyway When the pulls are equal there will be no current through this instru is we can replace _S_ by some other battery, say _X_, which ish to co for that battery in the sa 14 while the setting for _S_ was at _b_ We can see at once that _X_ is stronger than _S_ The question, however, is how er

Perhaps it would be better to try to answer this question by talking about e m f's It isn't fair to speak only of the positive plate which calls, weelectrons away from itself The idea of e m f takes care of both these actions The steady stream of electrons in the fine wire is due to the e m f of the battery _B_, that is to the pull of the positive terative

If the wire is uniforth, then each inch of it requires just as much e m f as any other inch Two inches require twice the e m f which one inch requires We knoin the part of the wire from _n_ to _b_ It takes just the e m f of the standard cell, _S_, because when that had its feet braced at _n_ it pulled just as hard at _b_ as did the big battery _B_

Suppose the distance _n_ to _d_ (usually written _nd_) is twice as great as that from _n_ to _b_ (_nb_) That means that battery _X_ has twice the e m f of battery _S_ You reth of wire _nd_, as could the large battery That is that cell _S_ can do Therefore if we kno many volts to call the e m

f of the standard cell we can say that _X_ has an e m f of twice as many volts

If we measured dry batteries this e should find that they each had an e e battery would be found to have about 24 volts when fully charged and perhaps as low as 21 volts e had run it for a while