way 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.

[Illustration: Fig 29]

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 met 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