ELECTRIC LIGHTING
In the first year of the nineteenth century one of the greatest of
England’s scientists, Sir Humphry Davy, became lecturer on chemistry
to the Royal Institution, where his brilliant lectures attracted large
and enthusiastic audiences. He was an indefatigable experimenter, and
in order to help on his work the Institution placed at his disposal
a very large voltaic battery consisting of 2000 cells. In 1802 he
found that if two rods of carbon, one connected to each terminal of
his great battery, were first made to touch one another and then
gradually separated, a brilliant arch of light was formed between them.
The intense brilliance of this electric arch, or _arc_ as it came to
be called, naturally suggested the possibility of utilizing Davy’s
discovery for lighting purposes, but the maintaining of the necessary
current proved a serious obstacle. The first cost of a battery of the
required size was considerable, but this was a small matter compared
with the expense of keeping the cells in good working order. Several
very ingenious and more or less efficient arc lamps fed by battery
current were produced by various inventors, but for the above reason
they were of little use except for experimental purposes, and the
commercial success of the arc lamp was an impossibility until the
dynamo came to be a really reliable source of current. Since that time
innumerable shapes and forms of arc lamps have been devised, while the
use of such lamps has increased by leaps and bounds. To-day, wherever
artificial illumination on a large scale is required, there the arc
lamp is to be found.
When the carbon rods are brought into contact and then slightly
separated, a spark passes between them. Particles of carbon are torn
off by the spark and volatilized, and these incandescent particles
form a sort of bridge which is a sufficiently good conductor for the
current to pass across it from one rod to the other. When the carbons
are placed horizontally, the glowing mass is carried upwards by the
ascending currents of heated air, and it assumes the arch-like form
from which it gets its name. If the carbons are vertical the curve is
not produced, a more or less straight line being formed instead. The
electric arc may be formed between any conducting substances, but for
practical lighting purposes carbon is found to be most suitable.
Either continuous or alternating currents may be used to form the arc.
With continuous current, if the carbon rods are fully exposed to the
air, they gradually consume away, and minute particles of carbon are
carried across from the positive rod to the negative rod, so that the
former wastes at about twice the rate of the latter. The end of the
positive rod becomes hollowed out so as to resemble a little crater,
and the end of the negative rod becomes more or less pointed. The fact
that with continuous current the positive rod consumes away twice
as fast as the negative rod, may be taken advantage of to decrease
the cost of new carbons, by replacing the wasted positive rod with a
new one, and using the unconsumed portion of the old positive rod as
a new negative rod.[1] If alternating current is used, each rod in
turn becomes the positive rod, so that no crater is formed, and both
the carbons have the same shape and are consumed at the same rate. A
humming noise is liable to be produced by the alternating current arc,
but by careful construction of the lamp this noise is reduced to the
minimum.
[1] In actual practice the positive carbon is made double the
thickness of the negative, so that the two consume at about
the same rate.
If the carbons are enclosed in a suitable globe the rate of wasting is
very much less. The oxygen inside the globe becomes rapidly consumed,
and although the globe is not air-tight, the heated gases produced
inside it check the entrance of further supplies of fresh air as long
as the lamp is kept burning. When the light is extinguished, and the
lamp cools down, fresh air enters again freely.
Arc lamp carbons may be either solid or cored. The solid form is made
entirely of very hard carbon, while the cored form consists of a narrow
tube of carbon filled up with soft graphite. Cored carbons usually
burn more steadily than the solid form. In what are known as flame arc
lamps the carbons are impregnated with certain metallic salts, such as
calcium. These lamps give more light for the same amount of current.
The arc is long and flame-like, and usually of a striking yellow
colour, but it is not so steady as the ordinary arc.
[Illustration: FIG. 21.--Diagram showing simple method of carbon
regulation for Arc Lamps.]
As the carbon rods waste away, the length of the arc increases, and
if this increase goes beyond a certain limit the arc breaks and the
current ceases. If the arc is to be kept going for any length of time
some arrangement for pushing the rods closer together must be provided,
in order to counteract the waste. In arc lamps this pushing together,
or “feeding” as it is called, is done automatically, as is also the
first bringing together and separating of the rods to start or strike
the arc. Fig. 21 shows a simple arrangement for this purpose. A is the
positive carbon, and B the negative. C is the holder for the positive
carbon, and this is connected to the rod D, which is made of soft
iron. This rod is wound with two separate coils of wire as shown,
coil E having a low resistance, and coil F a high one. These two coils
are solenoids, and D is the core, (Chapter VII.). When the lamp is not
in use, the weight of the holder keeps the positive carbon in contact
with the negative carbon. When switched on, the current flows along the
cable to the point H. Here it has two paths open to it, one through
coil E to the positive carbon, and the other through coil F and back
to the source of supply. But coil E has a much lower resistance than
coil F, and so most of the current chooses the easier path through E,
only a small amount of current taking the path through the other coil.
Both coils are now magnetized, and E tends to draw the rod D upwards,
while F tends to pull it downwards. Coil E, however, has much greater
power than coil F, because a much larger amount of current is passing
through it; and so it overcomes the feeble pull of F, and draws up the
rod. The raising of D lifts the positive carbon away from the negative
carbon, and the arc is struck. The carbons now begin to waste away,
and very slowly the distance between them increases. The path of the
current passing through coil E is from carbon A to carbon B by way of
the arc, and as the length of the gap between A and B increases, the
resistance of this path also increases. The way through coil E thus
becomes less easy, and as time goes on more and more current takes the
alternative path through coil F. This results in a decrease in the
magnetism of E, and an increase in that of F, and at a certain point F
becomes the more powerful of the two, and pulls down the rod. In this
way the positive carbon is lowered and brought nearer to the negative
carbon. Directly the diminishing distance between A and B reaches
a certain limit, coil E once more asserts its superiority, and by
overcoming the pull of F it stops the further approach of the carbons.
So, by the opposing forces of the two coils, the carbons are maintained
between safe limits, in spite of their wasting away.
[Illustration: PLATE IX.]
[Illustration:
_By permission of_ _Union Electric Co. Ltd._
NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS.]
The arc lamp is largely used for the illumination of wide streets,
public squares, railway stations, and the exteriors of theatres,
music-halls, picture houses, and large shops. The intense brilliancy
of the light produced may be judged from the accompanying photographs
(Plate IX.), which were taken entirely by the light of the arc lamps.
Still more powerful arc lamps are constructed for use in lighthouses.
The illuminating power of some of these lamps is equal to that of
hundreds of thousands of candles, and the light, concentrated by large
reflectors, is visible at distances varying from thirty to one hundred
miles.
Arc lamps are also largely used for lighting interiors, such as large
showrooms, factories or workshops. For this kind of lighting the
dazzling glare of the outdoor lamp would be very objectionable and
harmful to the eyes, so methods of indirect lighting are employed to
give a soft and pleasant light. Most of the light in the arc lamp comes
from the positive carbon, and for ordinary outdoor lighting this carbon
is placed above the negative carbon. In lamps for interior lighting
the arrangement is frequently reversed, so that the positive carbon is
below. Most of the light is thus directed upwards, and if the ceiling
is fairly low and of a white colour the rays are reflected by it, and
a soft and evenly diffused lighting is the result. Some light comes
also from the negative carbon, and those downward rays are reflected to
the ceiling by a reflector placed beneath the lamp. Where the ceiling
is very high or of an unsuitable colour, a sort of artificial ceiling
in the shape of a large white reflector is placed above the lamp to
produce the same effect. Sometimes the lamp is arranged so that part of
the light is reflected to the ceiling, and part transmitted directly
through a semi-transparent reflector below the lamp. The composition of
the light of the arc lamp is very similar to that of sunlight, and by
the use of such lamps the well-known difficulty of judging and matching
colours by artificial light is greatly reduced. This fact is of great
value in drapery establishments, and the arc lamp has proved a great
success for lighting rooms used for night painting classes.
The powerful searchlights used by warships are arc lamps provided with
special arrangements for projecting the light in any direction. A
reflector behind the arc concentrates the light and sends it out as a
bundle of parallel rays, and the illuminating power is such that a good
searchlight has a working range of nearly two miles in clear weather.
According to the size of the projector, the illumination varies from
about 3000 to 30,000 or 40,000 candle-power. For some purposes, such
as the illuminating of narrow stretches of water, a wider beam is
required, and this is obtained by a diverging lens placed in front
of the arc. In passing through this lens the light is dispersed or
spread out to a greater or less extent according to the nature of the
lens. Searchlights are used in navigating the Suez Canal by night,
for lighting up the buoys along the sides of the canal. The ordinary
form of searchlight does this quite well, but at the same time it
illuminates equally an approaching vessel, so that the pilot on this
vessel is dazzled by the blinding glare. To avoid this dangerous state
of things a split reflector is used, which produces two separate beams
with a dark space between them. In this way the sides of the canal are
illuminated, but the light is not thrown upon oncoming vessels, so that
the pilots can see clearly.
Glass reflectors are much more efficient than metallic ones, but they
have the disadvantage of being easily put out of action by gunfire.
This defect is remedied by protecting the glass reflector by a screen
of wire netting. This is secured at the back of the reflector, and
even if the glass is shattered to a considerable extent, as by a rifle
bullet, the netting holds it together, and keeps it quite serviceable.
Reflectors protected in this way are not put out of action by even two
or three shots fired through them. Searchlight arcs and reflectors
are enclosed in metal cylinders, which can be moved in any direction,
vertically or horizontally.
In the arc lamps already described, a large proportion of the light
comes from the incandescent carbon electrodes. About the year 1901 an
American electrician, Mr. P. C. Hewitt, brought out an arc lamp in
which the electrodes took no part in producing the light, the whole
of which came from a glowing stream of mercury vapour. This lamp,
under the name of the Cooper-Hewitt mercury vapour lamp, has certain
advantages over other electric illuminants, and it has come into
extensive use.
[Illustration: FIG. 22.--Sketch of Mercury Vapour Lamp.]
It consists of a long glass tube, exhausted of air, and containing a
small quantity of mercury. Platinum wires to take the current from
the source of supply are sealed in at each end. The tube is attached
to a light tubular framework of metal suspended from the ceiling, and
this frame is arranged so that it can be tilted slightly downwards by
pulling a chain. As shown in Fig. 22, the normal position of the lamp
is not quite horizontal, but tilted slightly downwards towards the end
of the tube having the bulb containing the mercury. The platinum wire
at this end dips into the mercury, so making a metallic contact with
it. The lamp is lighted by switching on the current and pulling down
the chain. The altered angle makes the mercury flow along the tube
towards the other platinum electrode, and as soon as it touches this a
conducting path for the current is formed from end to end of the tube.
The lamp is now allowed to fall back to its original angle, so that the
mercury returns to its bulb. There is now no metallic connexion between
the electrodes, but the current continues to pass through the tube as
a vacuum discharge. Some of the mercury is immediately vaporized and
rendered brilliantly incandescent, and so the light is produced. The
trouble of pulling down the chain is avoided in the automatic mercury
vapour lamp, which is tilted by an electro-magnet. This magnet is
automatically cut out of circuit as soon as the tilting is completed
and the arc struck.
The average length of the tube in the ordinary form of mercury vapour
lamp is about 30 inches, and a light of from 500 to 3000 candle-power
is produced, according to the current used. Another form, known as the
“Silica” lamp, is enclosed in a globe like that of an ordinary electric
arc lamp. The tube is only about 5 or 6 inches in length, and it is
made of quartz instead of glass, the arrangements for automatically
tilting the tube being similar to those in the ordinary form of lamp.
The light of the mercury vapour lamp is different from that of all
other lamps. Its peculiarity is that it contains practically no red
rays, most of the light being yellow, with a certain proportion of
green and blue. The result is a light of a peacock-blue colour. The
absence of red rays alters colour-values greatly, scarlet objects
appearing black; and on this account it is impossible to match colours
by this light. In many respects, however, the deficiency in red rays is
a great positive advantage. Every one who has worked by mercury vapour
light must have noticed that it enables very fine details to be seen
with remarkable distinctness. This property is due to an interesting
fact. Daylight and ordinary artificial light is a compound or mixture
of rays of different colours. It is a well-known optical fact that a
simple lens is unable to bring all these rays to the same focus; so
that if we sharply focus an image by red light, it is out of focus or
blurred by blue light. This defect of the lens is called “chromatic
aberration.” The eye too suffers from chromatic aberration, so that
it cannot focus sharply all the different rays at the same time. The
violet rays are brought to a focus considerably in front of the red
rays, and the green and the yellow rays come in between the two. The
eye therefore automatically and unconsciously effects a compromise,
and focuses for the greenish-yellow rays. The mercury vapour light
consists very largely of these rays, and consequently it enables the
image to be focused with greater sharpness; or, in other words, it
increases the acuteness of vision. Experiments carried out by Dr.
Louis Bell and Dr. C. H. Williams demonstrated this increase in visual
sharpness very conclusively. Type, all of exactly the same size, was
examined by mercury vapour light, and by the light from an electric
incandescent lamp with tungsten filament. The feeling of sharper
definition produced by the mercury vapour light was so strong that
many observers were certain that the type was larger, and they were
convinced that it was exactly the same only after careful personal
examination.
Mercury vapour light apparently imposes less strain upon the eyes than
ordinary artificial light, and this desirable feature is the result of
the absence of the red rays, which, besides having little effect in
producing vision, are tiring to the eyes on account of their heating
action. The light is very highly actinic, and for this reason it is
largely used for studio and other interior photographic work. In cases
where true daylight colour effects are necessary, a special fluorescent
reflector is used with the lamp. By transforming the frequency of the
light waves, this reflector supplies the missing red and orange rays,
the result being a light giving normal colour effects.
Another interesting vapour lamp may be mentioned briefly. This has a
highly exhausted glass tube containing neon, a rare gas discovered
by Sir William Ramsay. The light of this lamp contains no blue rays,
and it is of a striking red colour. Neon lamps are used chiefly for
advertising purposes, and they are most effective for illuminated
designs and announcements, the peculiar and distinctive colour of the
light attracting the eye at once.
An electric current meets with some resistance in passing through any
substance, and if the substance is a bad conductor the resistance is
very great. As the current forces its way through the resistance, heat
is produced, and a very thin wire, which offers a high resistance, may
be raised to a white heat by an electric current, and it then glows
with a brilliant light. This fact forms the basis of the electric
incandescent or glow lamp.
In the year 1878, Thomas A. Edison set himself the task of producing
a perfect electric incandescent lamp, which should be capable of
superseding gas for household and other interior lighting. The first
and the greatest difficulty was that of finding a substance which
could be formed into a fine filament, and which could be kept in a
state of incandescence without melting or burning away. Platinum was
first chosen, on account of its very high melting-point, and the fact
that it was not acted upon by the gases of the air. Edison’s earliest
lamps consisted of a piece of very thin platinum wire in the shape of a
spiral, and enclosed in a glass bulb from which the air was exhausted.
The ends of the spiral were connected to outside wires sealed into
the bulb. It was found, however, that keeping platinum continuously
at a high temperature caused it to disintegrate slowly, so that the
lamps had only a short life. Fine threads or filaments of carbon were
then tried, and found to be much more durable, besides being a great
deal cheaper. The carbon filament lamp quickly became a commercial
success, and up to quite recent years it was the only form of electric
incandescent lamp in general use.
In 1903 a German scientist, Dr. Auer von Welsbach, of incandescent gas
mantle fame, produced an electric lamp in which the filament was made
of the metal osmium, and this was followed by a lamp using the metal
tantalum for the filament, the invention of Siemens and Halske. For a
while the tantalum lamp was very successful, but more recently it has
been superseded in popularity by lamps having a filament of the metal
tungsten. The success of these lamps has caused the carbon lamp to
decline in favour. The metal filaments become incandescent much more
easily than the carbon filament, and for the same candle-power the
metal filament lamp consumes much less current than the carbon lamp.
The construction of tungsten lamps is very interesting. Tungsten is a
very brittle metal, and at first the lamps were fitted with a number
of separate filaments. These were made by mixing tungsten powder with
a sort of paste, and then squirting the mixture through very small
apertures, so that it formed hair-like threads. Early in 1911 lamps
having a filament consisting of a single continuous piece of drawn
tungsten wire were produced. It had been known for some time that
although tungsten was so brittle at ordinary temperatures, it became
quite soft and flexible when heated to incandescence in the lamp, and
that it lost this quality again as soon as it cooled down. A process
was discovered by which the metal could be made permanently ductile, by
mechanical treatment while in the heated state. In this process pure
tungsten powder is pressed into rods and then made coherent by heating.
While still hot it is hammered, and finally drawn out into fine wires
through diamond dies. The wire is no thicker than a fine hair, and it
varies in size from about 0·012 mm. to about 0·375 mm., according to
the amount of current it is intended to take. It is mounted by winding
it continuously zigzag shape round a glass carrier, which has at the
top and the bottom a number of metal supports arranged in the form
of a star, and insulated by a central rod of glass. One star is made
of strong, stiff material, and the other consists of fine wires of
some refractory metal, molybdenum being used in the Osram lamps. These
supports act as springs, and keep the wire securely in its original
shape, no matter in what position the lamp is used. The whole is placed
in a glass bulb, which is exhausted of air and sealed up.
For some purposes lamps with specially small bulbs are required, and in
these the tungsten wire is made in the shape of fine spirals, instead
of in straight pieces, so that it takes up much less room. In the
“Axial” lamp the spiral is mounted in such a position that most of the
light is sent out in one particular direction.
The latest development in electric incandescent lamps is the
“half-watt” lamp. The watt is the standard of electrical energy, and
it is the rate of work represented by a current of one ampere at a
pressure of 1 volt. With continuous currents the watts are found very
simply by multiplying together the volts and the amperes. For instance,
a dynamo giving a current of 20 amperes at a pressure of 50 volts would
be called a 1000-watt dynamo. With alternating currents the calculation
is more complicated, but the final result is the same. The ordinary
form of tungsten lamp gives about one candle-power for every watt, and
is known as a one-watt lamp. As its name suggests, the half-watt lamp
requires only half this amount of energy to give the same candle-power,
so that it is very much more economical in current. In this lamp the
tungsten filament is wound in a spiral, but instead of being placed in
the usual exhausted bulb, it is sealed into a bulb containing nitrogen
gas. The increased efficiency is obtained by running the filament at a
temperature from 400° to 600° C. higher than that at which the filament
in the ordinary lamp is used.
In spite of the great advances in artificial lighting made during
recent years, no one has yet succeeded in producing light without
heat. This heat is not wanted, and it represents so much waste energy.
It has often been said that the glow-worm is the most expert of all
illuminating engineers, for it has the power of producing at will a
light which is absolutely without heat. Perhaps the nearest approach
to light without heat is the so-called “cold light” invented by M.
Dussaud, a French scientist. His device consists of a revolving ring
of exactly similar tungsten lamps. Each of these lamps has current
passed through it in turn, and the duration of the current in each is
so short, being only a fraction of a second, that the lamp has not
sufficient time to develop any appreciable amount of heat. The light
from the ring of lamps is brought to a focus, and passed through a
lens to wherever it is required. Electric incandescent lamps are made
in a variety of sizes, each one being intended for a certain definite
voltage. If a lamp designed for, say, 8 volts, is used on a circuit
of 32 volts, its candle-power is greatly increased, while the amount
of current consumed is not increased in proportion. In this way the
lamp becomes a more efficient source of light, but the “over-running,”
as it is called, has a destructive effect on the filament, so that
the life of the lamp is greatly shortened. In the Dussaud system
however the time during which each lamp has current passing through
it is so short, followed by a period of rest, that the destructive
effect of over-running is reduced to the minimum; so that by using
very high voltages an extremely brilliant light is safely obtained
with a comparatively small consumption of current. It might be thought
that the constant interchange of lamps would result in an unsteady
effect, but the substitution of one lamp for another is carried out
so rapidly that the eye gets the impression of perfect steadiness.
The Dussaud system is of little use for ordinary lighting purposes,
but for lighthouse illumination, photographic studio work, and the
projection of lantern slides and cinematograph films, it appears to be
of considerable value.
Electric light has many advantages over all other illuminants. It gives
off very little heat, and does not use up the oxygen in the air of a
room as gas does; while by means of flexible wires the lamps can be put
practically anywhere, so that the light may be had just where it is
wanted. Another great advantage is that the light may be switched on
without any trouble about matches, and there is none of the danger from
fire which always exists with a flame.
The current for electric lamps is generally taken from the public
mains, but in isolated country houses a dynamo has to be installed
on the premises. This is usually driven by a small engine running on
petrol or paraffin. In order to avoid having to run the engine and
dynamo continually, the current is not taken directly from the dynamo,
but from a battery of accumulators. During the day the dynamo is used
to charge the accumulators, and these supply the current at night
without requiring any attention.
Electric lighting from primary cells is out of the question if a
good light is wanted continuously for long periods, for the process
is far too costly and troublesome. If a light of small candle-power
is required for periods of from a few minutes to about an hour, with
fairly long intervals of rest, primary cells may be made a success.
Large dry cells are useful for this purpose, but probably the most
satisfactory cell is the sack Leclanché. This is similar in working to
the ordinary Leclanché cell used for bells, but the carbon mixture is
placed in a canvas bag or sack, instead of in a porous pot, and the
zinc rod is replaced by a sheet of zinc surrounding the sack. These
cells give about 1½ volt each, so that four, connected in series,
are required to light a 6-volt lamp. The lamps must take only a very
small current, or the cells will fail quickly. Small metal filament
lamps taking from a third to half an ampere are made specially for this
purpose, and these always should be used. A battery of sack Leclanché
cells with a miniature lamp of this kind forms a convenient outfit
for use as a night-light, or for lighting a dark cupboard, passage
or staircase. Lamps with ruby glass, or with a ruby cap to slip over
the bulb, may be obtained for photographic purposes. If the outfit is
wanted for use as a reading-lamp it is better to have two separate
batteries, and to use them alternately for short periods. With this
arrangement each battery has a short spell of work followed by a rest,
and the light may be kept on for longer periods without overworking the
cells.