your receiving set must not only be connected with the aerial wire,
but with a wire that leads to and makes good contact with the moist
earth of the ground. Where a house or a building is piped for gas,
water or steam, it is easy to make a ground connection, for all you
have to do is to fasten the wire to one of the pipes with a clamp.
[Footnote: Pipes are often insulated from the ground, which makes them
useless for this purpose.] Where the house is isolated then a lot of
wires or a sheet of copper or of zinc must be buried in the ground at
a sufficient depth to insure their being kept moist.

About the Receiving Apparatus.--You can either buy the parts of the
receiving apparatus separate and hook them up yourself, or you can buy
the apparatus already assembled in a set which is, in the beginning,
perhaps, the better way.

The simplest receiving set consists of (1) a _detector_, (2) a _tuning
coil_, and (3) a _telephone receiver_ and these three pieces of
apparatus are, of course, connected together and are also connected to
the aerial and ground as the diagram in Fig. 1 clearly shows. There
are two chief kinds of detectors used at the present time and these
are: (a) the _crystal detector_, and (b) the _vacuum tube detector_.
The crystal detector is the cheapest and simplest, but it is not as
sensitive as the vacuum tube detector and it requires frequent
adjustment. A crystal detector can be used with or without a battery
while the vacuum tube detector requires two small batteries.

[Illustration: Fig. 1.--Simple Receiving Set.]

A tuning coil of the simplest kind consists of a single layer of
copper wire wound on a cylinder with an adjustable, or sliding,