ke the telephones act as if they had four times as
much resistance and so were exactly suited to be connected to the
audion.

This whole matter of the proper use of transformers is quite simple but
very important in setting up vacuum-tube circuits. To overlook it in
building or buying your radio set will mean poor efficiency. Whenever
you have two parts of a vacuum-tube circuit to connect together be sure
and buy only a transformer which is designed to work between the two
impedances (or resistances) which you wish to connect together.

There is one more precaution in connection with the purchase of
transformers. They should do the same thing for all the important
frequencies which they are to transmit. If they do not, the speech or
signals will be distorted and may be unintelligible.

If you take the precautions which I have mentioned your radio receiving
set formed by a detector and one amplifier will look like that of Fig.
94. That is only one possible scheme of connections. You can use any
detector circuit which you wish,[10] one with a grid condenser and leak,
or one arranged for feed-back In either case your amplifier may well be
as shown in the figure.

[Illustration: Fig 100]

The circuit I have described uses an audion to amplify the
audio-frequency currents which come from the detector and are capable of
operating the telephones. In some cases it is desirable to amplify the
radio signals before applying them to the detector. This is especially
true where a "loop antenna" is being used. Loop antennas are smaller and
more convenient than aerials and they also have certain abilities to
select the signals which they are to receive because they receive best
from stations which lie along a line drawn parallel to their turns.
Unfortunately, however, they are much less efficient and so require the
use of amplifiers.

With a small loop made by ten turns of wire separated by about a quarter
of an inch and wound on a square mounting, about three feet on a side,
you will usually require two amplifiers. One of these might be used to
amplify the radio signals before detection and the other to amplify
after detection. To tune the loop for broadcasts a condenser of about
0.0005 mf. will be needed. The diagram of Fig. 100 shows the complete
circuit of a set with three stages of radio-amplification and none of
audio.

[Footnote 10: Except for patented circuits. See p. 224.]

LETTER 20

TELEPHONE RECEIVERS AND