HOW LIGHT IS PRODUCED BY THE INCANDESCENT LAMP.
[Illustration: Fig. 192.]
[Illustration: Fig. 193.]
=148. Incandescence.= We have just seen that the electric current
produces heat when it flows through a conductor that offers
considerable resistance to it. As soon as this was discovered men
began to experiment to find whether a practical light could also be
produced. It was found that a wire could be kept hot by constantly
passing a current through it, and that the light given out from it
became whiter and whiter as the wire became hotter. The wire was said
to be _incandescent_, or glowing with heat. As metal wires are good
conductors of electricity, they had to be made extremely fine to offer
enough resistance; too fine, in fact, to be properly handled.
=149. The Incandescent Lamp.= Many substances were experimented upon
to find a proper material out of which could be made a _filament_
that would give the proper resistance and at the same time be strong
and lasting. It was found that hair-like pieces of carbon offered the
proper resistance to the current. When heated in the air, however,
carbon burns; so it became necessary to place the carbon filaments in a
globe from which all the air had been pumped before passing the current
through them. This proved to be a success.
[Illustration: Fig. 194.]
[Illustration: Fig. 195.]
[Illustration: Fig. 196.]
Fig. 192 shows the ordinary form of lamp. The _carbon filament_ is
attached, by carbon paste, to short platinum wires that are sealed in
the glass, their lower ends being connected to short copper wires that
are joined to the terminals of the lamp. When the lamp is screwed
into its socket, the current can pass up one side of the filament
and down the other. The filaments used have been made of every form
of carbonized vegetable matter. Bamboo has been largely used, fine
strips being cut by dies and then heated in air-tight boxes containing
fine carbon until they were thoroughly carbonized. This baking of the
bamboo produces a tough fiber of carbon. Various forms of thread have
been carbonized and used. Filaments are now made by pressing finely
pulverized carbon, with a binding material, through small dies. The
filaments are made of such sizes and lengths that will adapt them to
the particular current with which they are to be used. The longer the
filament, the greater its resistance, and the greater the voltage
necessary to push the current through it.
[Illustration: Fig. 197.]
[Illustration: Fig. 198.]
After the filaments are properly attached, the air is pumped from the
bulb or globe. This is done with some form of mercury pump, and the air
is so thoroughly removed from the bulb that about one-millionth only of
the original air remains. Before sealing off the lamp, a current is
passed through the filament to drive out absorbed air and gases, and
these are carried away by the pump. By proper treatment the filaments
have a uniform resistance throughout, and glow uniformly when the
current passes.
[Illustration: Fig. 199.]
[Illustration: Fig. 200.]
=150. Candle-Power.= A lamp is said to have 4, 8, 16 or more
candle-power. A 16-candle-power lamp, for example, means one that will
give as much light as sixteen standard candles. A standard sperm candle
burns two grains a minute. The candle-power of a lamp can be increased
by forcing a strong current through it, but this shortens its life.
_The Current_ used for incandescent lamps has to be strong enough to
force its way through the filament and produce a heat sufficient to
give a good light. The usual current has 50 or 110 volts, although
small lamps are made that can be run by two or three cells. If the
voltage of the current is less than that for which the lamp was made,
the light will be dim. The filament can be instantly burned out by
passing a current of too high pressure through it.
Even with the proper current, lamps soon begin to deteriorate, as small
particles of carbon leave the filament and cling to the glass. This is
due to the evaporation, and it makes the filament smaller, and a higher
pressure is then needed to force the current through the increased
resistance; besides this, the darkened bulb does not properly let the
light out. The current may be direct or alternating.
[Illustration: Fig. 201.]
[Illustration: Fig. 202.]
=151. The Uses= to which incandescent lamps are put are almost
numberless. Fig. 193 shows a decorative lamp. Fancy lamps are made in
all colors. Fig. 194 shows a conic candle lamp, to imitate a candle.
What corresponds to the body of the candle (see figure B to C) is a
delicately tinted opal glass tube surmounted (see figure A to B) by a
finely proportioned conic lamp with frosted globe. C to D in the figure
represents the regular base, and thus the relative proportions of the
parts are shown. Fig. 195 shows another form of candelabra lamp. Fig.
196 shows small dental lamps. Fig. 197 shows a small lamp with mirror
for use in the throat. Fig. 198 shows lamp with half shade attached,
used for library tables. Fig. 199 shows an electric pendant for several
lamps, with shade. Fig. 200 shows a lamp guard. Fig. 201 shows a lamp
socket, into which the lamp is screwed. Fig. 202 shows incandescent
bulbs joined in parallel to the + and - mains. Fig. 203 shows how the
lamp cord can be adjusted to desired length. Fig. 204 shows a lamp
with reflector placed on a desk. Fig. 205 shows a form of shade and
reflector.
[Illustration: Fig. 203.]
[Illustration: Fig. 204.]
[Illustration: Fig. 205.]