any in their
installations of house phones. You can also use a so-called "vacuum
lightning arrester." In either case the connections will be as shown in
Fig. 111. If you use a loop antenna, of course, no arrester is needed.

At first I would plan to receive signals between 150 meters and 360
meters. This will include the amateurs who work between 160 and 200 m.,
the special amateurs who send C-W telegraph at 275 m., and the
broadcasting stations which operate at 360 m. This range will give you
plenty to listen to while you are experimenting. In addition you will
get some ship signals at 300 m.

[Illustration: Fig 111]

To tune the antenna to any of the wave lengths in this range you can use
a coil of 75 turns wound on a cardboard tube of three and a half inches
in diameter. You can wind this coil of bare wire if you are careful,
winding a thread along with the wire so as to keep the successive turns
separated. In that case you will need to construct a sliding contact for
it. That is the simplest form of tuner.

On the other hand you can wind with single silk covered wire and bring
out taps at the 0, 2, 4, 6, 8, 10, 14, 20, 28, 36, 44, 56, 66, and 75th
turns. To make a tap drill a small hole through the tube, bend the wire
into a loop about a foot long and pull this loop through the hole as
shown in Fig. 110. Then give the wire a twist, as shown, so that it
can't pull out, and proceed with your winding.

Use 26 s. s. c. wire. You will need about 80 feet and might buy 200 to
have enough for the secondary coil. Make contacts to the taps by two
rotary switches as shown in Fig. 112. You can buy switch arms and
contacts studs or a complete switch mounted on a small panel of some
insulating compound. Let switch _s_{1}_ make the contacts for taps
between 14 and 75 turns, and let switch _s_{2}_ make the other
contacts.

For the secondary coil use the same size of wire and of core. Wind 60
turns, bringing out a tap at the middle. To tune the secondary circuit
you will need a variable condenser. You can buy one of the small ones
with a maximum capacity of about 0.0003 mf., one of the larger ones with
a maximum capacity of 0.0005 mf., or even the larger size which has a
maximum capacity of 0.001 mf. I should prefer the one of 0.0005 mf.

You will need a crystal detector--I should try galena first--and a
so-called "cat's whisker" with which to make contact with the galena.
For these parts and for the switch mentioned above y