cies of the two e. m. f.'s which are being
simultaneously impressed on the grid of the detector. Suppose the
incoming signal has a frequency of 100,000 cycles a second but that the
detector tube is oscillating in its own circuit at the rate of 99,000
cycles per second, then the listener will hear a note of 1000 cycles per
second. One thousand times each second the two e. m. f.'s will be
exactly in phase and one thousand times each second they will be exactly
opposite in phase. The voltage applied to the grid will be a maximum one
thousand times a second and alternately a minimum. We can think of it,
then, as if there were impressed on the grid of the detector a
high-frequency signal which varied in intensity one thousand times a
second. This we know will produce a corresponding variation in the
current through the telephone receiver and thus give rise to a musical
note of about two octaves above middle _C_ on the piano.

This circuit of Fig. 92 will let us detect signals which are not varying
in intensity. And consequently this is the method which we use to detect
the telegraph signals which are sent out by such a "continuous wave
transmitter" as I showed you at the end of Letter 13.

When the key of a C-W transmitter is depressed there is set up in the
distant receiving-antenna an alternating current. This current doesn't
vary in strength. It is there as long as the sender has his key down.
Because, however, of the effect which I described above there will be an
audible note from the telephone receiver if the detector tube is
oscillating at a frequency within two or three thousand cycles of that
of the transmitting station.

This method of receiving continuous wave signals is called the
"heterodyne" method. The name comes from two Greek words, "dyne" meaning
"force" and the other part meaning "different." We receive by combining
two different electron-moving-forces, one produced by the distant
sending-station and the other produced locally at the receiving station.
Neither by itself will produce any sound, except a click when it starts.
Both together produce a musical sound in the telephone receiver; and the
frequency of that note is the difference of the two frequencies.

There are a number of words used to describe this circuit with some of
which you should be familiar. It is sometimes called a "feed-back"
circuit because part of the output of the audion is fed back into its
input side. More generally it is known