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Incandescent Lamps
How Incandescent Lamps Work
The word incandescent means emitting visible radiation from solid material as the result of
being heated. Figure 4.1 shows that an incandescent lamp produces light from a length of heated
wire. The wire temperature can be as high as 2800 degrees Celsius. The wire is called a filament
and is normally in the shape of a coil. The wire is made of tungsten because of its very high
melting point.
Figure 4.1. Incandescent lamp
operation.
Lamp Parts When exposed to surrounding air, the heated filament literally burns up and
disintegrates in a short time. This is solved by enclosing the filament in a glass envelope or
BULB. See Figure 4.2. Air is removed from inside the bulb at the time of manufacture to form a
vacuum, or the bulb is filled with an inert gas usually a mixture of nitrogen and argon.
The LAMP CAP connects the lamp to the electrical circuit made of aluminium or brass.
FILAMENT SUPPORTS are wires that hold the filament in place - protect it during shipment and
in service.
LEAD WIRES are the internal wires that connect the filament to the lamp cap.
BUTTON is the top of central glass stem which holds the filament support wires.
HEAT DEFLECTING DISC (not shown) reduces the flow of hot gases from around the filament into
the neck of the bulb protects the stem and cap - usually only found in lamps of more than 250W.
PINCH is where the entry of the lead wires into the glass bulb are sealed.
EXHAUST TUBE is connected to a hole in the pinch and is where the air is removed and the
bulb filled with gas during manufacture.
FUSE is a protective device as part of the lead wire in case an arc or short circuit
develops inside the lamp.
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