n
parallel paths.

When we want a large inductance we wind the coil so that there are many
parallel paths. Then when the battery starts to drive an electron along,
this electron affects all its fellows who are in parallel paths and
tries to start them off in the opposite direction to that in which it is
being driven. The battery, of course, starts to drive all the electrons,
not only those nearest its negative terminal but those all along the
wire. And every one of these electrons makes up for the fact that the
battery is driving it along by urging all its fellows in the opposite
direction.

It is not an exceptional state of affairs. Suppose a lot of boys are
being driven out of a yard where they had no right to be playing.
Suppose also that a boy can resist and lag back twice as much if some
other boy urges him to do so. Make it easy and imagine three boys. The
first boy lags back not only on his own account but because of the
urging of the other boys. That makes him three times as hard to start as
if the other boys didn't influence him. The same is true of the second
boy and also of the third. The result is the unfortunate property owner
has nine times as hard a job getting that gang started as if only one
boy were to be dealt with. If there were two boys it would be four times
as hard as for one boy. If there were four in the group it would be
sixteen times, and if five it would be twenty-five times. The difficulty
increases much more rapidly than the number of boys.

Now all we have to do to get the right idea of inductance is to think of
each boy as standing for the electrons in one turn of the coil. If there
are five turns there will be twenty-five times as much inductance, as
for a single turn; and so on. You see that we can change the inductance
of a coil very easily by changing the number of turns.

I'll tell you two things more about inductance because they will come in
handy. The first is that the inductance will be larger if the turns are
large circles. You can see that for yourself because if the circles were
very small we would have practically a straight wire.

The other fact is this. If that property owner had been an electrical
engineer and the boys had been electrons he would have fixed it so that
while half of them said, "Aw, don't go; he can't put you off"; the other
half would have said "Come on, let's get out." If he did that he would
have a coil without any inductance, that is, he would have