comparative disuse, or rather were almost unknown, in many countries
of Europe as late as the fifteenth century; when the cottage and
baronial hall were alike lit by the blazing torch fixed into an iron
sconce or bracket on the wall.
The rushlight, consisting of a peeled rush, coated by repeated dipping
into a vessel of melted fat, made a feeble effort to dispel the gloom
of long winter evenings. This was succeeded by the tallow and more
scientifically made wax candle, which last still maintains a certain
popularity.
How our grandmothers managed to “keep their eyes” as they worked at
stitching by the light of a couple of candles, whose advent was the
event of the evening, is now a mystery. To-day we feel aggrieved if
our lamps are not of many candle-power, and protest that our sight
will be ruined by what one hundred and fifty years ago would have
seemed a marvel of illumination. In the case of lighting necessity has
been the mother of invention. The tendency of modern life is to turn
night into day. We go to bed late and we get up late; this is perhaps
foolish, but still we do it. And, what is more, we make increasing use
of places, such as basements, underground tunnels, and “tubes,” to
which the light of heaven cannot penetrate during any of the daily
twenty-four hours.
The nineteenth century saw a wonderful advance in the science of
illumination. As early as 1804 the famous scientist, Sir Humphrey
Davy, discovered the electric arc, presently to be put to such
universal use. About the same time gas was first manufactured and led
about in pipes. But before electricity for lighting purposes had been
rendered sufficiently cheap the discovery of the huge oil deposits in
Pennsylvania flooded the world with an inexpensive illuminant. As
early as the thirteenth century Marco Polo, the explorer, wrote of a
natural petroleum spring at Baku, on the Caspian Sea: “There is a
fountain of great abundance, inasmuch as a thousand shiploads might be
taken from it at one time. This oil is not good to use with food, but
it is _good to burn_; and is also used to anoint camels that have the
mange. People come from vast distances to fetch it, for in all other
countries there is no oil.” His last words have been confuted by the
American oil-fields, yielding many thousands of barrels a day--often
in such quantities that the oil runs to waste for lack of a buyer.
The rivals for pre-eminence in lighting to-day are electricity, coal
gas, petroleum, and acetylene gas. The two former have the advantage
of being easily turned on at will, like water; the third is more
generally available.
The invention of the dynamo by Gramme in 1870 marks the beginning of
an epoch in the history of illumination. With its aid current of such
intensity as to constantly bridge an air-gap between carbon points
could be generated for a fraction of the cost entailed by other
previous methods. Paul Jablochkoff devised in 1876 his “electric
candle”--a couple of parallel carbon rods separated by an insulating
medium that wasted away under the influence of heat at the same rate
as the rods. The “candles” were used with rapidly-alternating
currents, as the positive “pole” wasted twice as quickly as the
negative. During the Paris Exhibition of 1878 visitors to Paris were
delighted by the new method of illumination installed in some of the
principal streets and theatres.
The arc-lamp of to-day, such as we see in our streets, factories, and
railway stations, is a modification of M. Jablochkoff’s principle.
Carbon rods are used, but they are pointed towards each other, the
distance between their extremities being kept constant by ingenious
mechanical contrivances. Arc-lamps of all types labour under the
disadvantage of being, by necessity, very powerful; and were they only
available the employment of electric lighting would be greatly
restricted. As it is, we have, thanks to the genius of Mr. Edison, a
means of utilising current in but small quantities to yield a gentler
light. The glow-lamp, as it is called, is so familiar to us that we
ought to know something of its antecedents.
In the arc-lamp the electric circuit is _broken_ at the point where
light is required. In glow or incandescent lamps the current is only
_hindered_ by the interposition of a bad conductor of electricity,
which must also be incombustible. Just as a current of water flows in
less volume as the bore of a pipe is reduced, and requires that
greater pressure shall be exerted to force a constant amount through
the pipe, so is an electric current _choked_ by its conductor being
reduced in size or altered in nature. Edison in 1878 employed as the
current-choker a very fine platinum wire, which, having a melting
temperature of 3450 degrees Fahrenheit, allowed a very white heat to
be generated in it. The wire was enclosed in a glass bulb almost
entirely exhausted of air by a mercury-pump before being sealed. But
it was found that even platinum could not always withstand the heating
effect of a strong current; and accordingly Edison looked about for
some less combustible material. Mr. J. W. Swan of Newcastle-on-Tyne
had already experimented with carbon filaments made from cotton
threads steeped in sulphuric acid. Edison and Swan joined hands to
produce the present well-known lamp, “The Ediswan,” the filament of
which is a bamboo fibre, carbonised during the exhaustion of air in
the bulb to one-millionth of an atmosphere pressure by passing the
electric current through it. These bamboo filaments are very elastic
and capable of standing almost any heat.
Glow-lamps are made in all sizes--from tiny globes small enough to top
a tie-pin to powerful lamps of 1000 candle-power. Their independence
of atmospheric air renders them most convenient in places where other
forms of illumination would be dangerous or impossible; _e.g._ in coal
mines, and under water during diving operations. By their aid great
improvements have been effected in the lighting of theatres, which
require a quick switching on and off of light. They have also been
used in connection with minute cameras to explore the recesses of the
human body. In libraries they illuminate without injuring the books.
In living rooms they do not foul the air or blacken the ceiling like
oil or gas burners. The advantages of the “Edison lamp” are, in short,
multitudinous.
Cheapness of current to work them is, of course, a very important
condition of their economy. In some small country villages the
cottages are lit by electricity even in England, but these are
generally within easy reach of water power. Mountainous districts,
such as Norway and Switzerland, with their rushing streams and high
water-falls, are peculiarly suited for electric lighting: the cost of
which is mainly represented by the expense of the generating apparatus
and the motive power.
One of the greatest engineering undertakings in the world is connected
with the manufacture of electric current. Niagara, the “Thunder of the
Waters” as the Indians called it, has been harnessed to produce
electrical energy, convertible at will into motion, heat, or light.
The falls pass all the water overflowing from nearly 100,000 square
miles of lakes, which in turn drain a far larger area of territory.
Upwards of 10,000 cubic yards of water leap over the falls every
second, and are hurled downwards for more than 200 feet, with an
energy of eight or nine million horse-power! In 1886 a company
determined to turn some of this huge force to account. They bought up
land on the American bank, and cut a tunnel 6700 yards long, beginning
a mile and a half above the falls, and terminating below them. Water
drawn from the river thunders into the tunnel through a number of
wheel pits, at the bottom of each of which is a water-turbine
developing 5000 horse-power. The united force of the turbines is said
to approximate 100,000 horse-power; and as if this were but a small
thing, the same Company has obtained concessions to erect plant on the
Canadian bank to double or treble the total power.
So cheaply is current thus produced that the Company is in a position
to supply it at rates which appear small compared with those that
prevail in this country. A farthing will there purchase what would
here cost from ninepence to a shilling. Under such conditions the
electric lamp need fear no competitor.
But in less favoured districts gas and petroleum are again holding up
their heads.
Both coal and oil-gas develop a great amount of heat in proportion to
the light they yield. The hydrogen they contain in large quantities
burns, when pure, with an almost invisible flame, but more hotly than
any other known gas. The particles of carbon also present in the flame
are heated to whiteness by the hydrogen, but they are not sufficient
in number to convert more than a fraction of the heat into light.
A German, Auer von Welsbach, conceived the idea of suspending round
the flame a circular “mantle” of woven cotton steeped in a solution of
certain rare earths (_e.g._ lanthanum, yttrium, zirconium), to arrest
the heat and compel it to produce bright incandescence in the
arresting substance.
With the same gas consumption a Welsbach burner yields seven or more
times the light of an ordinary batswing burner. The light itself is
also of a more pleasant description, being well supplied with the blue
rays of the spectrum.
The mantle is used with other systems than the ordinary gas-jet.
Recently two methods of illumination have been introduced in which the
source of illumination is supplied under pressure.
The high-pressure incandescent gas installations of Mr. William Sugg
supply gas to burners at five or six times the ordinary pressure of
the mains. The effect is to pulverise the gas as it issues from the
nozzle of the burners, and, by rendering it more inflammable, to
increase its heating power until the surrounding mantle glows with a
very brilliant and white light of great penetration. Gas is forced
through the pipes connected with the lamps by hydraulic rams working
gas-pumps, which alternately suck in and expel the gas under a
pressure of twelve inches (_i.e._ a pressure sufficient to maintain a
column of water twelve inches high). The gas under this pressure
passes into a cylinder of a capacity considerably greater than the
capacity of the pumps. This cylinder neutralises the shock of the
rams, when the stroke changes from up-to downstroke, and _vice versâ_.
On the top of the cylinder is fixed a governor consisting of a strong
leathern gas-holder, which has a stroke of about three inches, and
actuates a lever which opens and closes the valve through which the
supply of water to the rams flows, and reduces the flow of the water
when it exceeds ten or twelve inches pressure, according to
circumstances. The gas-holder of the governor is lifted by the
pressure of the gas in the cylinder, which passes through a small
opening from the cylinder to the governor so as not to cause any
sudden rise or fall of the gas-holder. By this means a nearly constant
pressure is maintained; and from the outlet of the cylinder the gas
passes to another governor sufficient to supply the number of lights
the apparatus is designed for, and to maintain the pressure without
variation whether all or a few lamps are in action. For very large
installations steam is used.
Each burner develops 300 candle-power. A double-cylinder steam-engine
working a double pump supplies 300 of these burners, giving a total
lighting-power of 90,000 candles. As compared with the cost of
low-pressure incandescent lighting the high-pressure system is very
economical, being but half as expensive for the same amount of light.
It is largely used in factories and railway stations. It may be seen
on the Tower Bridge, Blackfriars Bridge, Euston Station, and in the
terminus of the Great Central Railway, St. John’s Wood.
Perhaps the most formidable rival to the electric arc-lamp for the
lighting of large spaces and buildings is the Kitson Oil Lamp, now so
largely used in America and this country.
The lamp is usually placed on the top of an iron post similar to an
ordinary gas-light standard. At the bottom of the post is a chamber
containing a steel reservoir capable of holding from five to forty
gallons of petroleum. Above the oil is an air-space into which air has
been forced at a pressure of fifty lbs. to the square inch, to act as
an elastic cushion to press the oil into the burners. The oil passes
upwards through an extremely fine tube scarcely thicker than electric
incandescent wires to a pair of cross tubes above the burners. The top
one of these acts as a filter to arrest any foreign matter that finds
its way into the oil; the lower one, in diameter about the size of a
lead-pencil and eight inches long, is immediately above the mantles,
the heat from which vaporises the small quantity of oil in the tube.
The oil-gas then passes through a tiny hole no larger than a
needle-point into an open mixing-tube where sufficient air is drawn in
for supporting combustion. The mixture then travels down to the
mantle, inside which it burns.
An ingenious device has lately been added to the system for
facilitating the lighting of the lamp. At the base of the lamp-post a
small hermetically-closed can containing petroleum ether is placed,
and connected by very fine copper-tubing with a burner under the
vaporising tube. When the lamp is to be lit a small rubber bulb is
squeezed, forcing a quantity of the ether vapour into the burner,
where it is ignited by a platinum wire rendered incandescent by a
current passing from a small accumulator also placed in the lamp-post.
The burner rapidly heats the vaporising tube, and in a few moments
oil-gas is passing into the mantles, where it is ignited by the
burner.
So economical is the system that a light of 1000 candle-power is
produced by the combustion of about half-a-pint of petroleum per hour!
Comparisons are proverbially odious, but in many cases very
instructive. Professor V. B. Lewes thus tabulates the results of
experiments with various illuminants:--
_Cost of 1000 candles per hour._
_s. d._
Electricity Per unit, 3-1/2d.
" Incandescent, 1 2
" Arc, 0 3-3/4
Coal-gas Flat flame, 1 6
" Incandescent, 0 2-1/4
" " high pressure, 0 1-3/4
Oil Lamp (oil at 8d. per gall.), 0 7-1/4
" Incandescent lamp, 0 2-1/4
" Kitson lamp, 0 1
Petroleum, therefore, at present comes in a very good first in
England.
The system that we have noticed at some length has been adapted for
lighthouse use, as it gives a light peculiarly fog-piercing. It is
said to approximate most closely to ordinary sunlight, and on that
account has been found very useful for the taking of photographs at
night-time. The portability of the apparatus makes it popular with
contractors; and the fact that its installation requires no tearing up
of the streets is a great recommendation with the long-suffering
public of some of our large towns.
Another very powerful light is produced by burning the gas given off
by carbide of calcium when immersed in water. _Acetylene_ gas, as it
is called, is now widely used in cycle and motor lamps, which emit a
shaft of light sometimes painfully dazzling to those who have to face
it. In Germany the gas is largely employed in village streets; and in
this country it is gaining ground as an illuminant of country houses,
being easy to manufacture--in small gasometers of a few cubic yards
capacity--and economical to burn.
Well supplied as we are with lights, we find, nevertheless, that
savants are constantly in pursuit of an _ideal_ illuminant.
From the sun are borne to us through the ether light waves, heat
waves, magnetic waves, and other waves of which we have as yet but a
dim perception. The waves are commingled, and we are unable to
separate them absolutely. And as soon as we try to copy the sun’s
effects as a source of heat or light we find the same difficulty. The
fire that cooks our food gives off a quantity of useless light-waves;
the oil-lamp that brightens one’s rooms gives off a quantity of
useless, often obnoxious, heat.
The ideal illuminant and the ideal heating agent must be one in which
the required waves are in a great majority. Unfortunately, even with
our most perfected methods, the production of light is accompanied by
the exertion of a disproportionate amount of wasted energy. In the
ordinary incandescent lamp, to take an instance, only 5 or 6 per cent.
of the energy put into it as electricity results in light. The rest is
dispelled in overcoming the resistance of the filament and agitating
the few air-molecules in the bulb. To this we must add the fact that
the current itself represents but a fraction of the power exerted to
produce it. The following words of Professor Lodge are to the point on
this subject:--
“Look at the furnaces and boilers of a steam-engine driving a group of
dynamos, and estimate the energy expended; and then look at the
incandescent filaments of the lamps excited by them, and estimate how
much of their radiated energy is of real service to the eye. It will
be as the energy of a pitch-pipe to an entire orchestra.
“It is not too much to say that a boy turning a handle could, if his
energy were properly directed, produce quite as much real light as is
produced by all this mass of mechanism and consumption of
material.”[6]
[6] Professor Oliver Lodge, in a lecture to the Ashmolean
Society, 3rd June 1889.
The most perfect light in nature is probably that of the glow-worm and
firefly--a phosphorescent or “cold” light, illuminating without
combustion owing to the absence of all waves but those of the
requisite frequency. The task before mankind is to imitate the
glow-worm in the production of isolated light-waves.
The nearest approach to its achievement has occurred in the
laboratories of Mr. Nikola Tesla, the famous electrician. By means of
a special oscillator, invented by himself, he has succeeded in
throwing the ether particles into such an intense state of vibration
that they become luminous. In other words, he has created vibrations
of the enormous rapidity of light, and this without the creation of
heat waves to any appreciable extent.
An incandescent lamp, mounted on a powerful coil, is lit _without_
contact by ether waves transmitted from a cable running round the
laboratory, or bulbs and tubes containing highly rarefied gases are
placed between two large plate-terminals arranged on the end walls. As
soon as the bulbs are held in the path of the currents passing through
the ether from plate to plate they become incandescent, shining with a
light which, though weak, is sufficiently strong to take photographs
by with a long exposure. Tesla has also invented what he calls a
“sanitary” light, as he claims for it the germ-killing properties of
sunshine. The lamps are glass tubes several feet long, bent into
spirals or other convolutions, and filled before sealing with a
certain gas. The ends of the glass tube are coated with metal and
provided with hooks to connect the lamp with an electric current. The
gas becomes _luminous_ under the influence of current, but not
strictly incandescent, as there is very little heat engendered. This
means economy in use. The lamps are said to be cheaply manufactured,
but as yet they are not “on the market.” We shall hear more of them in
the near future, which will probably witness no more interesting
development than that of lighting.
Before closing this chapter a few words may be said about new heating
methods. Gas stoves are becoming increasingly popular by reason of the
ease with which they can be put in action and made to maintain an even
temperature. But the most up-to-date heating apparatus is undoubtedly
electrical. Utensils of all sorts are fitted with very thin heating
strips (formed by the deposition of precious metals, such as gold,
platinum, &c., on exceedingly thin mica sheets), through which are
passed powerful currents from the mains. The resistance of the strip
converts the electromotive energy of the current into heat, which is
either radiated into the air or into water for cookery, &c.
In all parts of the house the electric current may be made to do work
besides that of lighting. It warms the passages by means of special
radiators--replacing the clumsy coal and “stuffy” gas stove; in the
kitchen it boils, stews, and fries, heats the flat-irons and ovens; in
the breakfast room boils the kettle, keeps the dishes, teapots, and
coffee-pots warm; in the bathroom heats the water; in the smoking-room
replaces matches; in the bedroom electrifies footwarmers, and--last
wonder of all--even makes possible an artificially warm bed-quilt to
heat the chilled limbs of invalids!
The great advantage of electric heating is the freedom from all smell
and smoke that accompanies it. But until current can be provided at
cheaper rates than prevail at present, its employment will be chiefly
restricted to the houses of the wealthy or to large establishments,
such as hotels, where it can be used on a sufficient scale to be
comparatively economical.
THE END
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