Letters of a Radio-Engineer to His Son

by John Mills

One thing this current-measuring instrument tells us is the direction of the electron stream through itself. It shows that the momentary stream of electrons goes through the coil from d to c, that is in the opposite direction to the stream in the part ab.

Now prepare to do a little close thinking. Read over carefully all I have told you about this experiment. You see that the moment the battery starts a stream of electrons from a towards b, something causes a momentary, that is a temporary, movement of electrons from d to c. We say that starting a 83 stream of electrons from a to b sets up or “induces” a stream of electrons from d to c.

What will happen then if we connect the battery between a and d as in Fig. 29? Electrons will start streaming away from a towards b, that is towards d. But that means there will be a momentary stream from d towards c, that is towards a. Our stream from the battery causes this oppositely directed stream. In the usual words we say it “induces” in the coil an opposing stream of electrons. This opposing stream doesn’t last long, as we saw, but while it does last it hinders the stream which the battery is trying to establish.

The stream of electrons which the battery causes will at first meet an opposition so it takes a little time before the battery can get the full-sized stream of electrons flowing steadily. In other words a current in a coil builds up slowly, because while it is building up it induces an effect which opposes somewhat its own building up.

Did you ever see a small boy start off somewhere, perhaps where he shouldn’t be going, and find his conscience starting to trouble him at once. For a time he goes a little slowly but in a moment or two his conscience stops opposing him and he goes on steadily at his full pace. When he started he stirred up his conscience and that opposed him. Nobody else was hindering his going. It was all brought about by his own actions. The opposition which he 84met was “self-induced.” He was hindered at first by a self-induced effect of his own conscience. If he was a stream of electrons starting off to travel around the coil we would 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 form and equally hard to break. It’s hard to get electrons going 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 “mil-henries” which are thousandths of a henry.

You ought to know what 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 stream 85which is increasing 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 more effect of inductance which we must know before we can get very far with our radio. Suppose an electron stream is flowing through a coil because a battery is driving 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 moment because the electrons have got the habit.