f the 5-ampere size will do. With these
you can arrange to make comparisons between different methods of
receiving. Suppose, for example, you connect the switches as shown in
Fig. 113 so that by throwing them to the left you are using the audion
and to the right the crystal as a detector. You can listen for a minute
in one position and then switch and listen for a minute in the other
position, and so on back and forth. That way you can tell whether or not
you really are getting better results.

If you want a rough measure of how much better the audion is than the
crystal you might see, while you are listening to the audion, how much
you can rob the telephone receiver of its current and still hear as well
as you do when you switch back to the crystal. The easiest way to do
this is to put a variable resistance across the receiver as shown in
Fig. 113. Adjust this resistance until the intensity of the signal when
detected by the audion is the same as for the crystal. You adjust this
variable resistance until it by-passes so much of the current, which
formerly went through the receiver, that the "audibility" of the signal
is reduced until it is the same as for the crystal detector. Carefully
made resistances for such a purpose are sold under the name of
"audibility meters." You can assemble a resistance which will do fairly
well if you will buy a small rheostat which will give a resistance
varying by steps of ten ohms from zero to one hundred ohms. At the same
time you can buy four resistance spools of one hundred ohms each and
perhaps one of 500 ohms. The spools need not be very expensive for you
do not need carefully adjusted resistances. Assemble them so as to make
a rheostat with a range of 0-1000 ohms by steps of 10 ohms. The cheapest
way to mount is with Fahnestock clips as illustrated in Fig. 114. After
a while, however, you will probably wish to mount them in a box with a
rotary switch on top.

[Illustration: Fig 114]

To study the effect of the grid condenser you can arrange switches so as
to insert this condenser and its leak and at the same time to cut out
the C-battery. Fig. 115 shows how. You can measure the gain in
audibility at the same time.

[Illustration: Pl. X.--Audio-frequency Transformer and Banked-wound Coil.
(Courtesy of Pacent Electric Co.)]

[Illustration: Fig 115]

After learning to use the audion as a detector, both by virtue of its
curved characteristic and by the grid-condenser method, I