I have explained how power can be converted into electric currents,
either continuous or alternating, and I must now show how these currents
can be applied to the production of light.
[Illustration: FIG. 15.]
The current may be used to produce an _arc light_ in the following
manner:--Two carbon rods, A and B, are held by suitable means in the
position shown in Fig. 15, and the two wires from a dynamo are joined
respectively to A and B, the upper one always being the positive lead
when a continuous current is used. When the current is sent through the
circuit, it passes through the carbons A and B, which are conductors.
Immediately this occurs, suitable mechanism in the lamp, being acted on
by the current, or by hand in the case of search-lights, or by
clock-work, moves the two carbons a small distance apart, with the
consequence that a dazzling arc of light is formed between them. If the
carbons get too far apart, the mechanism brings them nearer together
again, and on the delicacy with which it acts, depends the steadiness of
the light. It would be useless to explain how this mechanism acts, as it
is in a different form in each maker's lamp. Some lamps have been
constructed for use with an alternating current, but with the majority a
continuous current is used. While an arc light is burning the carbons
waste away, the upper one more rapidly than the lower, and the mechanism
has to approach them constantly to make up for this waste.
When the carbons are consumed as far as convenient, an automatic
arrangement cuts off the current, and the light goes out; or it diverts
the current to another set of carbons, which at once light up. The
carbons are made in suitable lengths to last a certain number of hours,
four, six, eight, &c. In Fig. 16 is shown an arc lamp complete.
[Illustration: FIG. 16. Arc Lamp Complete.]
An arc lamp is of necessity a complicated affair, which it is not
advisable to have on board ship, except where an electrician is engaged
permanently.
Another way of producing light is to use the current in what is called
an _electric candle_, of which a familiar type is the Jablochkoff
candle.
Fig. 17 shows the form of this candle, A and B being two carbon rods
parallel to one another, and joined, but at the same time insulated from
one another by kaolin, a sort of chalky substance, which is a
non-conductor.
[Illustration: FIG. 17.]
The wires C and D from the dynamo are joined respectively to A and B
through metallic supports, as in an arc lamp, and when the current is
turned on it flows through C A and across by a small strip of carbon E
to B and D back to the dynamo. The strip E is only large enough to carry
the current across for a moment, and is immediately consumed, but an arc
of light is then formed between the carbons as in the arc lamp. As the
carbons consume, the kaolin in between burns away, just in the same
manner as, in an ordinary candle, the wick is consumed and the wax melts
and burns away, except that in the latter case the wax feeds the light,
whereas the kaolin is only used to keep the carbons the required
distance apart and the arc of light from running down them. It is
evident that the carbons must be consumed equally, for which reason use
must be made of the alternating current. Any unsteadiness that occurs in
the light produced is consequent on unsteadiness of the current, or
impurities in the carbons, &c., there being no mechanism of any kind
required. These candles do not give such a great light as arc lights,
but it is of the same nature in every way. Fig. 18 shows one of these
candles in its holder, from which can be seen how electrical contact is
made with the two carbons.
[Illustration: FIG. 18.]
If the current is interrupted in any way, and the light goes out, it
will not be produced again automatically, but requires a small piece of
carbon between the two carbons as a path for the current to pass across
as in the beginning.
A third form of electric light is produced by using the current in an
_incandescent lamp_.
To explain the action of an incandescent lamp, I must refer back to what
I said about wires getting heated by a current being passed through them
which was too large for their capacity. If two large wires are joined by
a small one, and a strong current is passed through the circuit, the
small wire rapidly gets red hot, and finally fuses. If this small wire
is contained in a globe from which the air is exhausted, when the
current is passed through it, it gets red, then white hot, and when
very brilliant gets fused. If, instead of wire, we have in the small
globe a thin filament of carbon, when the current is passed through, we
get a brilliant light which remains constant because the carbon does not
fuse, and it cannot burn away for want of air. Fig. 19 shows a Swan
lamp, and Fig. 20 an Edison lamp, both made on this principle.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
If in these lamps the vacuum were perfect, the carbon filament would
never get consumed; it is, however, impossible to get a perfect vacuum,
but the better it is, the longer will the filament last. Incandescent
lamps are the only ones that are suitable for house or ship lighting.
[Advantages of incandescent lamps for house and ship lighting.] They
give a yellowish light like a good gas-flame, they do not consume the
air of a room, they cause no smell, and only give out a very slight
heat. They are perfectly safe, because if the globe gets broken and
allows air to get in, the filament is instantly consumed, and the light
goes out. They can be put in all sorts of places where it would be
impossible to have any other lamps, such as near the ceiling, close to
curtains, in a room full of explosives or combustibles, and even under
water. They are not affected by wind; they can therefore be used under
punkahs, or near open windows, sky-lights, or ports, or in the open air.
These lamps can be used with either continuous or alternating currents,
but will probably last longer with the latter, because, when a
continuous current is used, particles of the carbon of the filament
appear to be conveyed from one end of the filament to the other,
reducing the thickness at the one end, until finally it breaks. This
evidently cannot occur with an alternating current, as the impulse in
one direction is counteracted by the following one in the opposite
direction. If the current used is of too high a tension for the lamps,
they will show an intensely brilliant light for a short time, but the
filament will soon be destroyed, and the lamp rendered useless.