The loop is attached at its ends to two short pieces of platinum wire,
which pass through the glass of the bulb and around which the glass is
fused. As platinum has almost exactly the same coefficient of
heat-expansion as glass, the wires do not cause the glass to crack.
The process of manufacture includes the preparation of the filament.
This is made from paper, silk, bamboo fibre, tamidine, q. v., or other
material. After shaping into the form of the filament the material is
carbonized at a high heat, while embedded in charcoal, or otherwise
protected from the air. The flashing process (see Flashing of
incandescent Lamp Carbons) may also be applied. The attachment to the
platinum wires is effected by a minute clamp or by electric soldering.
The loop is inserted and secured within the open globe, which the glass
blower nearly closes, leaving one opening for exhaustion.
The air is pumped out, perhaps first by a piston pump, but always at the
end by a mercurial air pump. (See Pump, Geissler--and others.) As the
exhaustion becomes high a current is passed through the carbons heating
them eventually to white heat so as to expel occluded gas. The occluded
gases are exhausted by the pump and the lamp is sealed by melting the
glass with a blowpipe or blast-lamp flame. For the exhaustion several
lamps are usually fastened together by branching glass tubes, and are
sealed off one by one.
The incandescent lamps require about 3.5 watts to the candle power, or
give about 12 sixteen-candle lamps to the horse power expended on them.
Generally incandescent lamps are run in parallel or on multiple arc
circuits. All that is necessary in such distribution systems is to
maintain a proper potential difference between the two leads across
which the lamps are connected. In the manufacture of lamps they are
brought to an even resistance and the proper voltage at which they
should be run is often marked upon them. This may be fifty volts and
upward. One hundred and ten volts is a very usual figure. As current one
ampere for a fifty-volt, or about one-half an ampere for a one hundred
and ten volt lamp is employed.
Lamp, Incandescent, Three Filament.
A three filament lamp is used for three phase currents. It has three
filaments whose inner ends are connected, and each of which has one
leading-in wire. The three wires are connected to the three wires of the
circuit. Each filament receives a current varying in intensity, so that
there is always one filament passing a current equal to the sum of the
currents in the other two filaments.
Lamp, Lighthouse.
A special type of arc light. It is adapted for use in a lighthouse
dioptric lantern, and hence its arc has to be maintained in the same
position, in the focus of the lenses. The lamps are so constructed as to
feed both carbons instead of only one, thereby securing the above
object.