ELECTRIC LIGHT AND HEAT.
The electric spark was, of course, familiar to the early
experimenters with electricity, but the electric light, as we know
it, was first discovered by Sir Humphrey Davy, the Cornish
philosopher, in the year 1811 or thereabout. With the magic of his
genius Davy transformed the spark into a brilliant glow by passing
it between two points of carbon instead of metal. If, as in figure
60, we twist the wires (+ and--) which come from a voltaic
battery, say of 20 cells, about two carbon pencils, and bring
their tips together in order to start the current, then draw them
a little apart, we shall produce an artificial or mimic star. A
sheet of dazzling light, which is called the electric arc, is seen
to bridge the gap. It is not a true flame, for there is little
combustion, but rather a nebulous blaze of silvery lustre in a
bluish veil of heated air. The points of carbon are white-hot, and
the positive is eaten away into a hollow or crater by the current,
which violently tears its particles from their seat and whirls
them into the fierce vortex of the arc. The negative remains
pointed, but it is also worn away about half as fast as the
positive. This wasting of the carbons tends to widen the arc too
much and break the current, hence in arc lamps meant to yield the
light for hours the sticks are made of a good length, and a self-
acting mechanism feeds them forward to the arc as they are slowly
consumed, thus maintaining the splendour of the illumination.
Many ingenious lamps have been devised by Serrin, Dubosq, Siemens,
Brockie, and others, some regulating the arc by clockwork and
electro-magnetism, or by thermal and other effects of the current.
They are chiefly used for lighting halls and railway stations,
streets and open spaces, search-lights and lighthouses. They are
sometimes naked, but as a rule their brightness is tempered by
globes of ground or opal glass. In search-lights a parabolic
mirror projects all the rays in any one direction, and in
lighthouses the arc is placed in the focus of the condensing
lenses, and the beam is visible for at least twenty or thirty
miles on clear nights. Very powerful arc lights, equivalent to
hundreds of thousands of candles, can be seen for 100 or 150
miles.
Figure 61 illustrates the Pilsen lamp, in which the positive
Carbon G runs on rollers rr through the hollow interior of two
solenoids or coils of wire MM' and carries at its middle a
spindle-shaped piece of soft iron C. The current flows through the
solenoid M on its way to the arc, but a branch or shunted portion
of it flows through the solenoid M', and as both of these
solenoids act as electromagnets on the soft iron C, each tending
to suck it into its interior, the iron rests between them when
their powers are balanced. When, however, the arc grows too wide,
and the current therefore becomes too weak, the shunt solenoid M'
gains a purchase over the main solenoid M, and, pulling the iron
core towards it, feeds the positive carbon to the arc. In this way
the balance of the solenoids is readjusted, the current regains
its normal strength, the arc its proper width, and the light its
brilliancy.
Figure 62 is a diagrammatic representation of the Brush arc lamp.
X and Y are the line terminals connecting the lamp in circuit. On
the one hand, the current splits and passes around the hollow
spools H H', thence to the rod N through the carbon K, the arc,
the carbon K', and thence through the lamp frame to Y. On the
other hand, it runs in a resistance fine-wire coil around the
magnet T, thence to Y. The operation of the lamp is as follows: K
and K' being in contact, a strong current starts through the lamp
energising H and H', which suck in their core pieces N and S,
lifting C, and by it the "washer-clutch" W and the rod N and
carbon K, establishing the arc. K is lifted until the increasing
resistance of the lengthening arc weakens the current in H H' and
a balance is established. As the carbons burn away, C gradually
lowers until a stop under W holds it horizontal and allows N to
drop through W, and the lamp starts anew. If for any reason the
resistance of the lamp becomes too great, or the circuit is
broken, the increased current through T draws up its armature,
closing the contacts M, thus short-circuiting the lamp through a
thick, heavy wire coil on T, which then keeps M closed, and
prevents the dead lamp from interfering with the others on its
line. Numerous modifications of this lamp are in very general use.
Davy also found that a continuous wire or stick of carbon could be
made white-hot by sending a sufficient current through it, and
this fact is the basis of the incandescent lamp now so common in
our homes.
Wires of platinum, iridium, and other inoxidisable metals raised
to incandescence by the current are useful in firing mines, but
they are not quite suitable for yielding a light, because at a
very high temperature they begin to melt. Every solid body becomes
red-hot--that is to say, emits rays of red light, at a temperature
of about 1000 degrees Fahrenheit, yellow rays at 1300 degrees,
blue rays at 1500 degrees, and white light at 2000 degrees. It is
found, however, that as the temperature of a wire is pushed beyond
this figure the light emitted becomes far more brilliant than the
increase of temperature would seem to warrant. It therefore pays
to elevate the temperature of the filament as high as possible.
Unfortunately the most refractory metals, such as platinum and
alloys of platinum with iridium, fuse at a temperature of about
3450 degrees Fahrenheit. Electricians have therefore forsaken
metals, and fallen back on carbon for producing a light. In 1845
Mr. Staite devised an incandescent lamp consisting of a fine rod
or stick of carbon rendered white-hot by the current, and to
preserve the carbon from burning in the atmosphere, he enclosed it
in a glass bulb, from which the air was exhausted by an air pump.
Edison and Swan, in 1878, and subsequently, went a step further,
and substituted a filament or fine thread of carbon for the rod.
The new lamp united the advantages of wire in point of form with
those of carbon as a material. The Edison filament was made by
cutting thin slips of bamboo and charring them, the Swan by
carbonising linen fibre with sulphuric acid. It was subsequently
found that a hard skin could be given to the filament by
"flashing" it--that is to say, heating it to incandescence by the
current in an atmosphere of hydrocarbon gas. The filament thus
treated becomes dense and resilient.
Figure 63 represents an ordinary glow lamp of the Edison-Swan
type, where E is the filament, moulded into a loop, and cemented
to two platinum wires or electrodes P penetrating the glass bulb
L, which is exhausted of air.
Platinum is chosen because it expands and contracts with
temperature about the same as glass, and hence there is little
chance of the glass cracking through unequal stress. The vacuum in
the bulb is made by a mercurial air pump of the Sprengel sort, and
the pressure of air in it is only about one-millionth of an
atmosphere. The bulb is fastened with a holder like that shown in
figure 64, where two little hooks H connected to screw terminals T
T are provided to make contact with the platinum terminals of the
lamp (P, figure 63), and the spiral spring, by pressing on the
bulb, ensures a good contact.
Fig. 65 is a cut of the ordinary Edison lamp and socket. One end
of the filament is connected to the metal screw ferule at the
base. The other end is attached to the metal button in the centre
of the extreme bottom of the base. Screwing the lamp into the
socket automatically connects the filament on one end to the
screw, on the other to an insulated plate at the bottom of the
socket.
The resistance of such a filament hot is about 200 ohms, and to
produce a good light from it the battery or dynamo ought to give
an electromotive force of at least 100 volts. Few voltaic cells or
accumulators have an electromotive force of more than 2 volts,
therefore we require a battery of 50 cells joined in series, each
cell giving 2 volts, and the whole set 100 volts. The strength of
current in the circuit must also be taken into account. To yield a
good light such a lamp requires or "takes" about 1/2 an ampere.
Hence the cells must be chosen with regard to their size and
internal resistance as well as to their kind, so that when the
battery, in series, is connected to the lamp, the resistance of
the whole circuit, including the filament or lamp, the battery
itself, and the connecting wires shall give by Ohm's law a current
of 1\2 an ampere. It will be understood that the current has the
same strength in every part of the circuit, no matter how it is
made up. Thus, if 1/2 of an ampere is flowing in the lamp, it is
also flowing in the battery and wires. An Edison-Swan lamp of this
model gives a light of about 15 candles, and is well adapted for
illuminating the interior of houses. The temperature of the carbon
filament is about 3450 degrees Fahr--that is to say, the
temperature at which platinum melts. Similar lamps of various
sizes and shapes are also made, some equivalent to as many as 100
candles, and fitted for large halls or streets, others emitting a
tiny beam like the spark of a glow-worm, and designed for medical
examinations, or lighting flowers, jewels, and dresses in theatres
or ball-rooms.
The electric incandescent lamp is pure and healthy, since it
neither burns nor pollutes the air. It is also cool and safe, for
it produces little heat, and cannot ignite any inflammable stuffs
near it. Hence its peculiar merit as a light for colliers working
in fiery mines. Independent of air, it acts equally well under
water, and is therefore used by divers. Moreover, it can be fixed
wherever a wire can be run, does not tarnish gilding, and lends
itself to the most artistic decoration.
Electric lamps are usually connected in circuit on the series,
parallel, and three wire system.
The series system is shown in figure 66, where the lamps L L
follow each other in a row like beads on a string. It is commonly
reserved for the arc lamp, which has a resistance so low that a
moderate electromotive force can overcome the added resistance of
the lamps, but, of course, if the circuit breaks at any point all
the lamps go out.
The parallel system is illustrated in figure 67, where the lamps
are connected between two main conductors cross-wise, like the
steps of a ladder. The current is thus divided into cross
channels, like water used for irrigating fields, and it is obvious
that, although the circuit is broken at one point, say by the
rupture of a filament, all the lamps do not go out.
Fig. 68 exhibits the Edison three-wire system, in which two
batteries or dynamos are connected together in series, and a third
or central main conductor is run from their middle poles. The plan
saves a return wire, for if two generators had been used
separately, four mains would have been necessary.
The parallel and three-wire systems in various groups, with or
without accumulators as local reservoirs, are chiefly employed for
incandescent lamps.
The main conductors conveying the current from the dynamos are
commonly of stout copper insulated with air like telegraph wires,
or cables coated with india-rubber or gutta-percha, and buried
underground or suspended overhead. The branch and lamp conductors
or "leads" are finer wires of copper, insulated with india-rubber
or silk.
The current of an installation or section of one is made and
broken at will by means of a "switch" or key turned by hand. It is
simply a series of metal contacts insulated from each other and
connected to the conductors, with a sliding contact connected to
the dynamo which travels over them. To guard against an excess of
current on the lamps, "cut-outs," or safety-fuses, are inserted
between the switch and the conductors, or at other leading points
in the circuit. They are usually made of short slips of metal foil
or wire, which melt or deflagrate when the current is too strong,
and thus interrupt the circuit.
There is some prospect of the luminosity excited in a vacuum tube
by the alternating currents from a dynamo or an induction coil
becoming an illuminant. Crookes has obtained exquisitely beautiful
glows by the phosphorescence of gems and other minerals in a
vacuum bulb like that shown in figure 69, where A and B are the
metal electrodes on the outside of the glass. A heap of diamonds
from various countries emit red, orange, yellow, green, and blue
rays. Ruby, sapphire, and emerald give a deep red, crimson, or
lilac phosphorescence, and sulphate of zinc a magnificent green
glow. Tesla has also shown that vacuum bulbs can be lit inside
without any outside connection with the current, by means of an
apparatus like that shown in figure 70, where D is an alternating
dynamo, C a condenser, P S the primary and secondary coils of a
sparking transformer, T T two metal sheets or plates, and SB the
exhausted bulbs. The alternating or see-saw current in this case
charges the condenser and excites the primary coil P, while the
induced current in the secondary coil 5 charges the terminal
plates T T. So long as the bulbs or tubes are kept within the
space between the plates, they are filled with a soft radiance,
and it is easy to see that if these plates covered the opposite
walls of a room, the vacuum lamps would yield a light in any part
of it.
Electric heating bids fair to become almost as important as
electric illumination. When the arc was first discovered it was
noticed that platinum, gold, quartz, ruby, and diamond--in fine,
the most refractory minerals--were melted in it, and ran like wax.
Ores and salts of the metals were also vapourised, and it was
clear that a powerful engine of research had been placed in the
hands of the chemist. As a matter of fact, the temperature of the
carbons in the arc is comparable to that of the Sun. It measures
5000 to 10,000 degrees Fahrenheit, and is the highest artificial
heat known. Sir William Siemens was among the first to make an
electric furnace heated by the arc, which fused and vapourised
metallic ores, so that the metal could be extracted from them.
Aluminium, chromium, and other valuable metals are now smelted by
its means, and rough brilliants such as those found in diamond
mines and meteoric stones have been crystallised from the fumes of
carbon, like hoar frost in a cold mist.
The electric arc is also applied to the welding of wires, boiler
plates, rails, and other metal work, by heating the parts to be
joined and fusing them together.
Cooking and heating by electricity are coming more and more into
favour, owing to their cleanliness and convenience. Kitchen
ranges, including ovens and grills, entirely heated by the
electric current, are finding their way into the best houses and
hotels. Most of these are based on the principle of incandescence,
the current heating a fine wire or other conductor of high
resistance in passing through it. Figure 71 represents an electric
kettle of this sort, which requires no outside fire to boil it,
since the current flows through fine wires of platinum or some
highly resisting metal embedded in fireproof insulating cement in
its bottom. Figures 72 and 73 are a sauce-pan and a flat-iron
heated in the same way. Figure 74 is a cigar-lighter for smoking
rooms, the fusee F consisting of short platinum wires, which
become red-hot when it is unhooked, and at the same time the lamp
Z is automatically lit. Figure 75 is an electric radiator for
heating rooms and passages, after the manner of stoves and hot
water pipes. Quilts for beds, warmed by fine wires inside, have
also been brought out, a constant temperature being maintained by
a simple regulator, and it is not unlikely that personal clothing
of the kind will soon be at the service of invalids and chilly
mortals, more especially to make them comfortable on their
travels.
An ingenious device places an electric heater inside a hot water
bag, thus keeping it at a uniform temperature for sick-room and
hospital use.