tact with them. Then we have an
inductance the value of which we can vary "step-by-step" in a convenient
manner.

Another way to make a variable inductance is to make what is called a
"variometer." I dislike the name because it doesn't "meter" anything. If
properly calibrated it would of course "meter" inductance, but then it
should be called an "inducto-meter."

Do you remember the gang of boys that fellow had to drive off his
property? What if there had been two different gangs playing there? How
much trouble he has depends upon whether there is anything in common
between the gangs. Suppose they are playing in different parts of his
property and so act just as if the other crowd wasn't also trespassing.
He could just add the trouble of starting one gang to the trouble of
starting the other.

It would be very different if the gangs have anything in common. Then
one would encourage the other much as the various boys of the same gang
encourage each other. He would have a lot more trouble. And this extra
trouble would be because of the relations between gangs, that is,
because of their "mutual inductance."

On the other hand suppose the gangs came from different parts of the
town and disliked each other. He wouldn't have nearly the trouble. Each
gang would be yelling at the other as they went along: "You'd better
beat it. He knows all right, all right, who broke that bush down by the
gate. Just wait till he catches you." They'd get out a little easier,
each in the hope the other crowd would catch it from the owner. There's
a case where their mutual relations, their mutual inductance, makes the
job easier.

That's true of coils with inductance. Suppose you wind two inductance
coils and connect them in series. If they are at right angles to each
other as in Fig. 46a they have no effect on each other. There is no
mutual inductance. But if they are parallel and wound the same way like
the coils of Fig. 46b they will act like a single coil of greater
inductance. If the coils are parallel but wound in opposite directions
as in Fig. 46c they will have less inductance because of their mutual
inductance. You can check these statements for yourself if you'll refer
back to Letter 10 and see what happens in the same way as I told you in
discussing Fig. 28.

[Illustration: Fig 46a]

[Illustration: Fig 46b]

If the coils are neither parallel nor at right angles there will be some
mutual inductance but not as much as if they