FIGURE. A single-filament bulb.
A lamp generates light through a process of changing energy forms called incandescence. The lamp produces light as a result of current flow through a filament. The filament is enclosed within a glass envelope and is a type of resistance wire that is generally made from tungsten. As current flows through the tungsten filament, it gets very hot. The changing of electrical energy to heat energy in the resistive wire filament is so intense that the filament starts to glow and emits light. The lamp must have a vacuum surrounding the filament to prevent it from burning so hot that the filament burns in two. The glass envelope that encloses the filament maintains the presence of vacuum. When the lamp is manufactured, all the air is removed and the glass envelope seals out the air. If air is allowed to enter the lamp, the oxygen would cause the filament to oxidize and burn up.
FIGURE. A double-filament lamp.
Many lamps are designed to execute more than one function. A double-filament lamp has two filaments so the bulb can perform more than one function. It can be used in the stop light circuit, taillight circuit, and turn signal circuit combined.
It is important that any burned-out lamp be replaced with the correct lamp. The technician can determine what lamp to use by checking the lamp’s standard trade number.
TABLE. A TABLE OF SOME TYPICAL AUTOMOTIVE LIGHT BULB EXAMPLES.