ight make the e. m. f.
stronger, that is pull and shove the electrons harder, but unless the
coil has a very small inductance or unless the frequency is very low we
should have to use an e. m. f. of enormous strength to get any
appreciable current.

Condensers are just the other way in their action. If there is a
condenser in a circuit, where an alternating e. m. f. is active, there
is lots of trouble if the frequency is low. If, however, the frequency
is high the same-sized current can be maintained by a smaller e. m. f.
than if the frequency is low. You see, when the frequency is high the
electrons hardly get into the waiting-room of the condenser before it is
time for them to turn around and go toward the other room. Unless there
is a large current, there are not enough electrons crowded together in
the waiting-room to push back very hard on the next one to be sent along
by the e. m. f. Because the electrons do not push back very hard a small
e. m. f. can drive them back and forth.

Ordinarily we say that a condenser impedes an alternating current less
and less the higher is the frequency of the current. And as to
inductances, we say that an inductance impedes an alternating current
more and more the higher is the frequency.

Now we are ready to study the receiving circuit of Fig. 54. I showed you
in Fig. 57 how the current through, the tube will vary as time goes on.
It increases and decreases with the frequency of the current in the
antenna of the distant transmitting station. We have a picture, or
graph, as we say, of how this plate current varies. It will be necessary
to study that carefully and to resolve it into its components, that is
to separate it into parts, which, added together again will give the
whole. To show you what I mean I am going to treat first a very simple
case involving money.

Suppose a boy was started by his father with 50 cents of spending money.
He spends that and runs 50 cents in debt. The next day his father gives
him a dollar. Half of this he has to spend to pay up his yesterday's
indebtedness. This he does at once and that leaves him 50 cents ahead.
But again he buys something for a dollar and so runs 50 cents in debt.
Day after day this cycle is repeated. We can show what happens by the
curve of Fig. 61a.

[Illustration: Fig 61a]

On the other hand, suppose he already had 60 cents which, he was saving
for some special purpose. This he doesn't touch, preferring to run into
debt