stream of any size going in
circuit _L-C_. It is equally hard if _L-C_ has smaller capacity and
inductance than _ab_ so that it naturally oscillates faster.

I'll tell you exactly what it is like. Suppose you and your partner are
trying to dance without any piano or other source of music. She has one
tune running through her head and she dances to that, except as you drag
her around the floor. You are trying to follow another tune. As a couple
you have a difficult time going anywhere under these conditions. But it
would be all right if you both had the same tune.

If we want the electron stream in coil _ab_ to have a large guiding
effect on the stream in coil _L-C_ we must see that both circuits
have the same tune, that is the same natural frequency of oscillation.

[Illustration: Fig 51]

This can be shown very easily by a simple experiment. Suppose we set up
our circuit _L-C_ with an ammeter in it, so as to be able to tell
how large an electron stream is oscillating in that circuit. Let us also
make the condenser a variable one so that we can change the natural
frequency or tune of the circuit. Now let's see what happens to the
current as we vary this condenser, changing the capacity and thus
changing the tune of the circuit. If we use a variable plate condenser
it will have a scale on top graduated in degrees and we can note the
reading of the ammeter for each position of the movable plates. If we
do, we find one position of these plates, that is one setting,
corresponding to one value of capacity in the condenser, where the
current in the circuit is a maximum. This is the setting of the
condenser for which the circuit has the same tune or natural frequency
as the circuit _cd_. Sometimes we say that the circuits are now in
resonance. We also refer to the curve of values of current and condenser
positions as a "tuning curve." Such a curve is shown in Fig. 51.

[Illustration: Fig 52]

That's all there is to tuning--adjusting the capacity and inductance of
a circuit until it has the same natural frequency as some other circuit
with which we want it to work. We can either adjust the capacity as we
just did, or we can adjust the inductance. In that case we use a
variable inductance as in Fig. 52.

If we want to be able to tune to any of a large range of frequencies we
usually have to take out or put into the circuit a whole lot of
mil-henries at a time. When we do we get these mil-henries of inductance
from a coi