Part 5 (1/2)

That is the way in which to compare batteries and to measure their e m

f's, but you see it takes a lot of time It is easier to use a ”volt e m f's Here is how one could beinstrument which is quite sensitive, so that its pointer will show a deflection when only a very sh the instrument We could make one in the same way as we made the ammeter of the last letter but there are other better ways of which I'll tell you later Then we connect a good deal of fine wire in series with the instrument for a reason which I'll tell you in a minute The next and last step is to calibrate

We knothe electron stream between _n_ and _b_--we know that from the e

m f of our standard cell Suppose then that we connect this new instrument, which we have just15 Soet away froative plate of battery _B_ can now travel as far as _b_ through the wire of the new instrus around to some new position Opposite that we mark the number of volts which the standard battery told us there was between _n_ and _b_

[Illustration: Fig 15]

If we move the end of the wire from _b_ to _d_ the pointer will take a new position Opposite this we mark twice the number of volts of the standard cell We can run it to a point _e_ where the distance _ne_ is one-half _nb_, and mark our scale with half the number of volts of the standard cell, and so on for other positions along the wire That's the e calibrate a sensitive current- instrument (with its added wire, of course) so that it will read volts It is now a voltmeter

If we connect a volt 16 the pointer will tell us the number of volts in the e m f of _X_, for the pointer will take the same position as it did when the voltmeter was connected between _n_ and _d_

There is only one thing to watch out for in all this We must be careful that the voltmeter is so made that it won't offer too easy a path for electrons to follow We only want to find how hard a battery can pull an electron, for that is e mean by e m f Of course, we h the voltmeter so as to make the pointer move That is why volt piece of fine wire or else have a coil of fine wire in series with the current- and narrow path for the electrons and so there can be only a s that a volt 16]

Fine wires offer more resistance to electron streath If a wire is the sath of it which we use the greater is the resistance which is offered to an electron stream

You will need to kno to describe the resistance of a wire or of any part of an electric circuit To do so you tell how many ”ohms” of resistance it has The ohm is the unit in which we measure the resistance of a circuit to an electron stream

I can show you what an ohm is if I tell you a simple way to measure a resistance Suppose you have a wire or coil of wire and want to know its resistance Connect it in series with a battery and an a 17 The sah all parts of this circuit and the ammeter tells us what this stream is in amperes Now connect a volture

The volt applied to force the current through the coil Divide the number of volts by the number of amperes and the quotient (answer) is the number of oh 17]

Suppose the ammeter shows a current of one a 1 by 1 gives 1 That means that the coil has a resistance of one ohm It also means one ohh it a current of one a out of this For example, it means also that one volt will send a current of one ah a resistance of one ohm

How many ohms would the coil have if it took 5 volts to send 2 aet 25 Therefore the coil would have a resistance of 25 ohms

Try another If a coil of resistance three ohe across the terive a current of 1 ampere, wouldn't it? For 3 ohive twice this current would take twice 3 volts That is, 2 amperes in 3 ohms requires 2x3 volts

Here's one for you to try by yourself If an e h a resistance of 2 oh? Notice that I told the nu to tell? Don't tell just the number; tell how many and what

LETTER 9

THE AUDION-CHARACTERISTIC

MY DEAR YOUNG STUDENT:

Although there is much in Letters 7 and 8 which it is well to learn and to think about, there are only three of the ideas which you et theto write you about the audion

First: Electric currents are streams of electrons We measure currents in amperes To measure a current we may connect into the circuit an ammeter