The same electron stream passes through all parts of this
circuit and the ammeter tells us what this stream is in amperes. Now
connect a voltmeter to the two ends of the coil as shown in the figure.
The voltmeter tells in volts how much e. m. f. is being 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 ohms of resistance
in the coil.

[Illustration: Fig 17]

Suppose the ammeter shows a current of one ampere and the voltmeter an
e. m. f. of one volt. Then dividing 1 by 1 gives 1. That means that the
coil has a resistance of one ohm. It also means one ohm is such a
resistance that one volt will send through it a current of one ampere.
You can get lots of meaning out of this. For example, it means also that
one volt will send a current of one ampere through a resistance of one
ohm.

How many ohms would the coil have if it took 5 volts to send 2 amperes
through it. Solution: Divide 5 by 2 and you get 2.5. Therefore the coil
would have a resistance of 2.5 ohms.

Try another. If a coil of resistance three ohms is carrying two amperes
what is the voltage across the terminals of the coil? For 1 ohm it would
take 1 volt to give a current of 1 ampere, wouldn't it? For 3 ohms it
takes three times as much to give one ampere. To give 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. m. f. of 8 volts is
sending current through a resistance of 2 ohms, how much current is
flowing? Notice that I told the number of ohms and the number of volts,
what are you going 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 must have
firmly grasped to get the most out of this letter which I am now going
to 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.

Second: Electrons move in a circuit when there is an
electron-moving-force, that is an electromotive force or e. m. f. We
measure e. m. f.'s in volts. To measure an e. m. f. we connect a
voltmeter to the two points between which the e. m. f. is active.

Third: Wh