Incandescent Lamps 6

The incandescent lamp is a simple device—a hot wire (the filament) sealed in a

glass jar (the bulb).

An electric current passing through the wire
heats it to incandescence, and the wire emits
light. The filament wire diameter and length
determine the amount of electrical current
drawn by the lamp, regulating its light output
(figure 6.1).
The incandescent lamps discussed in
this chapter are commonly referred to as
“large” lamps. This designation does not
refer to large physical size, but has tradition-
ally described lamps that operate on stan-
dard-voltage circuits. The “large lamp”
category now includes lamps of many volt-
ages commonly found in residential, com-
mercial, and industrial use.
“Miniature” lamps, conversely, are not
necessarily small, although many of them
are. They are lamps that operate at less
common voltages, powered by storage bat-
teries or by transformers that reduce or
increase the standard voltage to the voltage
required by the lamp. Their predominant use
is in transportation vehicles and instruments.
The family of large lamps contains about
one hundred combinations of glass and
quartz bulb shapes and sizes. These varia-
tions are designated by a two-part abbrevia-

tion: the first part, one or more letters, indi-

cates the shape of the bulb; the second part,

a number, indicates the diameter of the bulb

in eighths of an inch. For example, an A19

lamp is anarbitrary-shaped lamp that is^198

(2^38 ) inches in diameter (figure 6.2).

Figure 6.1Incandescent lamp components.