single-wire systems. In this system, one side of the
lamp filament connects to the base and the other
to the contact. Double contact bases are used if
the single-wire system is not practicable. In this
system, the filament connects to contacts and does
not make an electrical connection to the base
unless the lamp has dual filaments. Dual filaments
use a common contact to the base. The single- and
double-contact lamps are not interchangeable.
Some bases are of the screw type. They aren't
used often because they loosen easily. Other lamps
have an indexing-type base that has offset index
pins. (See figure 4-1.) The index-type base is used
to make sure that the lamp sits in the socket and
the light shines in the proper direction.
Figure 4-2.-Grain of wheat lamp.
Some lamps do not have a base; they solder
directly onto a circuit board or permanently
mount in control box panels. These lamps go
bulb with a 6/8-inch diameter. A variety of sizes
through careful testing and selection so they last
and shapes of bulbs are listed in the Defense
the life of the aircraft. Figure 4-2 shows one such
Logistics Agency (DLA) Identification List. This
lamp, the grain of wheat lamp.
list is available through your local supply support
The bulb shape is shown by a combination of
center. Figure 4-3 shows some common bulb
letters and numerals. The letter shows the bulb
shapes.
shape, and the number shows the approximate
Most aviation lamps are either clear glass or
maximum diameter of the bulb, in eighths of an
frosted on the inside. For a particular application,
inch. The codes and the shapes they indicate for
however, a bulb may be partially frosted; for
the more common glass bulbs are as follows:
G
Globular
GG
Grimes globular
Straight
S
Tubular
T
PAR
Parabolic aluminum reflector
R
Reflector
By looking at the letter designations, you
Figure 4-3.-Common bulb shapes.
know that a bulb designated as T-6 is a tubular
Figure 4-1.-Lamp bases.
4-2
