were parallel. By turning
the coils we can get all the variations in mutual relations from the
case of Fig. 46b to that of Fig. 46c. That's what we arrange to do in a
variable inductance of the variometer type.

[Illustration: Fig 46c]

There is another way of varying the mutual inductance. We can make one
coil slide inside another. If it is way inside, the total inductance
which the two coils offer is either larger than the sum of what they can
offer separately or less, depending upon whether the windings are in the
same direction or opposite. As we pull the coil out the mutual effect
becomes less and finally when it is well outside the mutual inductance
is very small.

Now we have several methods of varying capacity and inductance and
therefore we are ready to vary the frequency of our audion oscillator;
that is, "tune" it, as we say. In my next letter I shall show you why we
tune.

Now for the rule which I promised. The frequency to which a circuit is
tuned depends upon the product of the number of mil-henries in the coil
and the number of microfarads in the condenser. Change the coil and the
condenser as much as you want but keep this product the same and the
frequency will be the same.

[Footnote 5: More accurately the number is 6,286,000,000,000.]

LETTER 13

TUNING

DEAR RADIO ENTHUSIAST:

I want to tell you about receiving sets and their tuning. In the last
letter I told you what determines the frequency of oscillation of an
audion oscillator. It was the condenser and inductance which you studied
in connection with Fig. 36. That's what determines the frequency and
also what makes the oscillations. All the tube does is to keep them
going. Let's see why this is so.

[Illustration: Fig 47a]

Start first, as in Fig. 47a, with a very simple circuit of a battery and
a non-inductive resistance, that is, a wire wound like that of Fig. 40
in the previous letter, so that it has no inductance. The battery must
do work forcing electrons through that wire. It has the ability, or the
energy as we say.

[Illustration: Fig 47b]

Now connect a condenser to the battery as in Fig. 47b. The connecting
wires are very short; and so practically all the work which the battery
does is in storing electrons in the negative plate of the condenser and
robbing the positive plate. The battery displaces a certain number of
electrons in the waiting-rooms of the condenser. How many, depends upon
how hard it can push an