t sending station. The speech
significance is now hidden in a current of a frequency intermediate
between radio and audio. This current may be amplified many times and
then supplied to the grid of a detector which obtains from it a current
of audio-frequency which has a speech significance. In Fig. 128 I have
indicated the several operations.

We can now see why this method permits sharper tuning. The whole idea
of tuning, of course, is to arrange that the incoming signal shall cause
the largest possible current and at the same time to provide that any
signals at other wave-lengths shall cause only negligible currents. What
we want a receiving set to do is to distinguish between two signals
which differ slightly in wave-length and to respond to only one of them.

Suppose we set up a tuned circuit formed by a coil and a condenser and
try it out for various frequencies of signals. You know how it will
respond from our discussion in connection with the tuning curve of Fig.
51 of Letter 13. We might find from a number of such tests that the best
we can expect any tuned circuit to do is to discriminate between signals
which differ about ten percent in frequency, that is, to receive well
the desired signal and to fail practically entirely to receive a signal
of a frequency either ten percent higher or the same amount lower.

For example, if the signal is at 30,000 cycles a tuned circuit might be
expected to discriminate against an interfering signal of 33,000. If the
signal is at 300,000 cycles a tuned circuit might discriminate against
an interfering signal of 330,000 cycles, but an interference at 303,000
cycles would be very troublesome indeed. It couldn't be "tuned out" at
all.

Now suppose that the desired signal is at 300,000 cycles and that there
is interference at 303,000 cycles. We provide a local oscillator of
270,000 cycles a second, receive by this "super-heterodyne" method which
I have just described, and so obtain an intermediate frequency. In the
output of the first detector we have then a current of 300,000--270,000
or 30,000 cycles due to the desired signal and also a current of
303,000--270,000 or 33,000 cycles due to the interference. Both these
currents we can supply to another tuned circuit which is tuned for
30,000 cycles a second. It can receive the desired signal but it can
discriminate against the interference because now the latter is ten
percent "off the tune" of the signal.

You see the questi