only the
natural inertia of the electrons to deal with. We would say that he had
made a coil with "pure resistance" or else that he had made a
"non-inductive resistance."

[Illustration: Fig 40]

How would he do it? Easy enough after one learns how, but quite
ingenious. Take the wire and fold it at the middle. Start with the
middle and wind the coil with the doubled wire. Fig. 40 shows how the
coil would look and you can see that part of the way the electrons are
going around the coil in one direction and the rest of the way in the
opposite direction. It is just as if the boys were paired off, a
"goody-goody" and a "tough nut" together. They both shout at once
opposite advice and neither has any effect.

I have told you all except one of the ways in which we can affect the
inductance of a circuit. You know now all the methods which are
important in radio. So let's consider how to make large or small
capacities.

First I want to tell you how we measure the capacity of a condenser. We
use units called "microfarads." You remember that an ampere means an
electron stream at the rate of about six billion billion electrons a
second. A millionth of an ampere would, therefore, be a stream at the
rate of about six million million electrons a second--quite a sizable
little stream for any one who wanted to count them as they went by. If a
current of one millionth of an ampere should flow for just one second
six million million electrons would pass along by every point in the
path or circuit.

That is what would happen if there weren't any waiting-rooms in the
circuit. If there was a condenser then that number of electrons would
leave one waiting-room and would enter the other. Well, suppose that
just as the last electron of this enormous number[5] entered its
waiting-room we should know that the voltage of the condenser was just
one volt. Then we would say that the condenser had a capacity of one
microfarad. If it takes half that number to make the condenser oppose
further changes in the contents of its waiting-rooms, with one volt's
worth of opposition, that is, one volt of e. m. f., then the condenser
has only half a microfarad of capacity. The number of microfarads of
capacity (abbreviated mf.) is a measure of how many electrons we can get
away from one plate and into the other before the voltage rises to one
volt.

What must we do then to make a condenser with large capacity? Either of
two things; either make the waiti