f.
to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of
wave-lengths from 270 to 860 m. Using half the coil the range is 150 to
480 m. With the larger condenser the ranges are respectively 270 to 1220
and 270 to 670. For longer wave-lengths load with inductance. Four times
the inductance will tune to double these wave-lengths.

[Footnote 11: If you can afford to buy, or if you can borrow, ammeters
and voltmeters of the proper range you should take the characteristic
yourself.]

LETTER 22

HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS

MY DEAR EXPERIMENTER:

This letter is to summarize the operations which must be performed in
radio-telephone transmission and reception; and also to describe the
circuit of an important commercial system.

To transmit speech by radio three operations are necessary. First, there
must be generated a high-frequency alternating current; second, this
current must be modulated, that is, varied in intensity in accordance
with the human voice; and third, the modulated current must be supplied
to an antenna. For efficient operation, of course, the antenna must be
tuned to the frequency which is to be transmitted. There is also a
fourth operation which is usually performed and that is amplification.
Wherever the electrical effect is smaller than desired, or required for
satisfactory transmission, vacuum tubes are used as amplifiers. Of this
I shall give you an illustration later.

Three operations are also essential in receiving. First, an antenna must
be so arranged and tuned as to receive energy from the distant
transmitting station. There is then in the receiving antenna a current
similar in wave form to that in the transmitting antenna. Second, the
speech significance of this current must be detected, that is, the
modulated current must be demodulated. A current is then obtained which
has the same wave form as the human voice which was the cause of the
modulation at the distant station. The third operation is performed by a
telephone receiver which makes the molecules of air in its neighborhood
move back and forth in accordance with the detected current. As you
already know a fourth operation may be carried on by amplifiers which
give on their output sides currents of greater strength but of the same
forms as they receive at their input terminals.

In transmitting and in receiving equipment two or more of these
operations may be performed by the same vacuum tube as you w