s, to rectify the incoming radio waves so
that they may be translated into sound by the telephone receivers.
Receiving sets using these crystals do not use a battery, but these
sets are not very sensitive, and cannot "pick up" weak waves. This
means that crystal receiving sets must be used near the broadcasting
stations, before the waves have been weakened by traveling any
considerable distance.

As a general rule, the radio-listener's first receiving set uses a
crystal detector. Very often it is difficult to obtain good results
with such a set, and a more elaborate set is obtained. Moreover, even
if a crystal set does give good results, the owner of such a set soon
hears of friends who are able to hear concerts sent out from distance
stations. This gives him the desire to be able to hear such stations
also and he then buys a receiving set which uses the "audion-bulb" for
detecting, or rectifying the incoming waves.

The audion-bulb resembles an ordinary incandescent lamp. It contains
three elements:

1. In the center of the bulb is a short tungsten filament, the ends of
which are brought out to two terminals in the base of the bulb. This
filament must be heated to incandescence, and a storage battery is
required for this purpose, because it is necessary to have a very
steady current in order to obtain clear sounds in the receiver. Lately
plans have been suggested for using a direct current lighting line,
and even an alternating current lighting line for heating the
filament, but at present such plans have not been perfected, and the
battery will undoubtedly continue to be used with the majority of sets.

2. Surrounding the filament but not touching it is a helix of wire,
only one end of which is brought out to a terminal in the base of the
bulb. This helix is called the "grid." In some bulbs the grid is not
made in the form of a helix, but is made of two flat gridlike
structures, one on each side of the filament.

3. Surrounding the "grid" is the "plate" which is sometimes in the
shape of a hollow metallic cylinder. Some plates are not round, but
may be oval, or they may be two flat plates joined together at some
point, and one placed on either side of the grid. The plate has one
terminal in the base of the bulb.

[Fig. 159 Illustrating the principle of the Audion Bulb]

The action of an audion-bulb is quite complex, but a simpler
explanation, though one which may not be exactly correct from a purely
technic