possible. All points of contact must be well insulated with
glass, porcelain, or hard rubber.

The tuning coil is very simple in construction. A cardboard tube,
about three inches in diameter, is mounted between two square heads.
This tube is wound with No. 24 insulated copper wire and very well
shellaced to avoid loosening of the wire.

Two pieces of one quarter inch square brass rod, to be fastened
between the heads, are secured, and a slider, as shown in drawing, is
made. The rods are fastened on the heads and the insulation in the
path of the slides is then well scraped off. Binding posts are then
fastened to rods and coil ends.

[Illustration: Details of instruments for field use.]

{213}

The detector, although the most important of the instruments, is
perhaps the simplest. It is constructed of a hardwood base with a
small brass plate fastened on by means of a binding post. On the other
end of the base is fastened a double binding post which holds a brass
spring, as in the drawing. On the end of this spring is fastened a
copper point made by winding a few inches of No. 36 or 40 wire on it
and allowing about three sixteenths of an inch to project. This
completes the detector but, for use in this instrument, lead sulphide
or Galena crystals must be secured.

The condenser is made of two pieces of tin-foil, four by ten, and
three pieces of waxed paper a little larger than the foil. A piece of
wire is twisted into the end of each piece of foil, and then one sheet
of foil is laid on a sheet of paper. This is then covered by another
sheet of paper upon which is laid the second sheet of foil. On top of
this is laid the third sheet of paper and the whole is folded into a
convenient bundle. The sheets of foil must be well insulated from each
other and the wires must project from the condenser.

The ground connection is made by soldering a wire to a cold water
pipe. In the case of a portable set the ground may be made by driving
a metal rod into the ground or sinking metal netting into a body of
water.

The telephone receivers cannot well be made and must therefore be
bought. The type of phones used will therefore depend entirely on the
builder's purse.

The Sending Set

The same aerial and ground are used for sending as were used for
receiving, and for the experimenter, it will be far cheaper to buy a
spark coil for his sending set than to attempt to make one.

For a field set there will be very little