Part 7 (1/2)

When he started he stirred up his conscience and that opposed hi It was all brought about by his own actions The opposition which he met was ”self-induced” He was hindered at first by a self-induced effect of his own conscience If he was a strea off to travel around the coil ould say that he was opposed by a self-induced e m f And any path in which such an effect will be produced we say has ”self-inductance”

Usually we shorten this term and speak of ”inductance”

There is another way of looking at it We know habits are hard to for around a coil and the self-inductance of a circuit tells us how hard it is The harder it is the more self-inductance we say that the coil or circuit has Of course, we need a unit in which to measure self-inductance The unit is called the ”henry” But that is more self-inductance than we can stand in most radio circuits, so we find it convenient to measure in smaller units called ”ht to knohat a henry[4] is, if we are to use the word, but it isn't necessary just now to spend much time on it The opposition which one's self-induced conscience offers depends upon how rapidly one starts It's volts which make electrons move and so the conscience which opposes them will be measured in volts Therefore we say that a coil has one henry of inductance when an electron strea one ampere's worth each second stirs up in the coil a conscientious objection of one volt Don't try to remember this now; you can come back to it later

There is one et very far with our radio Suppose an electron strea the electrons along Now let the battery be removed or disconnected You'd expect the electron stream to stop at once but it doesn't It keeps on for a ot the habit

[Illustration: Fig 28]

If you look again at Fig 28 you will see what I mean Suppose the switch is closed and a steady streah the coil from _a_ to _b_ There will be no current in the other part of the coil Now open the switch There will be ainstru a momentary current

The direction of this motion, however, shows that the h the coil from _c_ to _d_

Do you see what thisdriving the electrons in the part _ab_ and they slon Immediately, and just for an instant, a stream of electrons starts off in the part _cd_ in the sa

Now look again at Fig 29 If the battery is suddenly disconnected there is a momentary rush of electrons in the sa the of a strea of a strea 29]

So far we haven't saideit useful for radio-telephony Before radio was possible all these things that I have just told you, and soood scientists years of patient study and experiment to find out those ideas about electricity which have made possible radio-telephony

Two of these ideas are absolutely necessary for the student of radio-coap in a circuit where there are waiting-rooms for the electrons Second: Electrons forh a coil of wire, harder than through a straight wire, but after they are going they don't like to stop They like it ht wire

In my next letter I'll tell you what happens e have a coil and a condenser together in a circuit

[Footnote 4: The ”henry” has nothing to do with a well-known automobile

It was nao at Princeton University]

LETTER 11

A ”C-W” TRANSMITTER

DEAR SON:

[Illustration: Fig 28]

Let's look again at the coils of Fig 28 which we studied in the last letter I have reproduced them here so you won't have to turn back When electrons start from _a_ towards _b_ there is a momentary stream of electrons froh _ab_ were started in the opposite direction, that is from _b_ to _a_ the induced stream in the coil _cd_ would be fro 30]

It all ree or fence between the 30 One boy is after the other Suppose you were being chased; you knohat you'd do If your pursuer started off with a rush towards one end of the hedge you'd ”beat it” towards the other But if he started slowly and cautiously you would start slowly too You always go in the opposite direction, dodging back and forth along the paths which you are wearing in the grass on opposite sides of the hedge If he starts to the right and then slows up and starts back, you will start to your right, slow up, and start back Suppose he starts at the center of the hedge

First he dodges to the right, and then back through the center as far to the left, then back again and so on You follow his every change

[Illustration: Fig 31]

I a to make a picture of what you two do Let's start with the other fellow He dodges or alternates back and forth Some persons would say he ”oscillates” back and forth in the same path As he does so he induces you to -picture ca 31 shows the way the filht only your heads The white circle represents the tow-head onBroho lives next door Of course, the camera only catches you each time the shutter opens but it is easy to draw a co 32