the size of the plates. We can do that either by using larger
plates or other plates and connecting it as in Fig. 41 so that the total
waiting-room space for electrons is increased.

[Illustration: Pl. VI.--Low-power Transmitting Tube, U V 202
(Courtesy of Radio Corporation of America).]

[Illustration: Fig 42]

If you have got these ideas you can understand how we use both sides of
the same plate in some types of condensers. Look at Fig. 42. There are
two plates connected together and a third between them. Suppose
electrons are pulled from the outside plates and crowded into the middle
plate. Some of them go on one side and some on the other, as I have
shown. The negative signs indicate electrons and the plus signs their
old homes. If we use more plates as in Fig. 43 we have a larger
capacity.

[Illustration: Fig 43]

[Illustration: Fig 44]

What if we have two plates which are not directly opposite one another,
like those of Fig. 44? What does the capacity depend upon? Imagine
yourself an electron on the negative plate. Look off toward the positive
plate and see how big it seems to you. The bigger it looks the more
capacity the condenser has. When the plates are right opposite one
another the positive plate looms up pretty large. But if they slide
apart you don't see so much of it; and if it is off to one side about
all you see is the edge. If you can't see lots of atoms which have lost
electrons and so would make good homes for you, there is no use of your
staying around on that side of the plate; you might just as well be
trying to go back home the long way which you originally came.

That's why in a variable plate condenser there is very little capacity
when no parts of the plates are opposite each other, and there is the
greatest capacity when they are exactly opposite one another.

[Illustration: Fig 45]

While we are at it we might just as well clean up this whole business of
variable capacities and inductances by considering two ways in which to
make a variable inductance. Fig. 45 shows the simplest way but it has
some disadvantages which I won't try now to explain. We make a long coil
and then take off taps. We can make connections between one end of the
coil and any of the taps. The more turns there are included in the part
of the coil which we are using the greater is the inductance. If we want
to do a real job we can bring each of these taps to a little stud and
arrange a sliding or rotating con