he same time you
should buy the proper design of transformer to go between the plate
circuit of your tube and the pair of receivers which you have. It will
usually be advisable to ask the dealer to show you a characteristic
curve for the transformer, which will indicate how well the transformer
operates at the different frequencies in the audio range. It should
operate very nearly the same for all frequencies between 200 and 2500
cycles.

The next step is to learn to use the tube as a detector. Connect it
into your secondary circuit instead of the crystal detector. Use the
proper value of C-battery as determined from your study of the
characteristic of the tube. One or two small dry cells, which have
binding-post terminals are convenient C-batteries. If you think you
will need a voltage much different from that obtained with a whole
number of batteries you can arrange to supply the grid as we did in
Fig. 86 of Letter 18. In that case you can use a few feet of 30
German-silver wire and make connections to it with a suspender clip.
Learn to receive with the tube and be particularly careful not to let
the filament have too much current and burn out.

Now buy some more apparatus. You will need a grid condenser of about
0.0002 mf. The grid leaks to go with it you can make for yourself. I
would use a piece of brown wrapping paper and two little metal eyelets.
The eyelets can be punched into the paper. Between them coat the paper
with carbon ink, or with lead pencil marks. A line about an inch long
ought to serve nicely. You will probably wish to make several grid leaks
to try. When you get satisfactory operation in receiving by the
grid-condenser method the leak will probably be somewhere between a
megohm and two megohms.

For this method you will not want a C-battery, but you will wish to
operate the detector with about as high a voltage as the manufacturers
will recommend for the plate circuit. In this way the incoming signal,
which decreases the plate current, can produce the largest decrease. It
is also possible to start with the grid slightly positive instead of
being as negative as it is when connected to the negative terminal of
the A-battery. There will then be possible a greater change in grid
voltage. To do so connect the grid as in Fig. 115 to the positive
terminal of the A-battery.

[Illustration: Fig 113]

About this time I would shop around for two or three small double-pole
double-throw switches. Those o