mechanism for regulating the distance of the carbons. This invention is
known as the _electric candle_, and is of great interest from the fact
that it was with this arrangement that the electric light was, for the
first time, practically employed for street and theatre illumination.
This was in 1878, when visitors to Paris, during the Exhibition, were
astonished by the splendid displays in the Avenue de l’Opéra, at the
shops of the Louvre, and at some of the theatres. Then it was shown, for
the first time, that electric lighting was not merely a scientific
curiosity, but a new and formidable rival to gas. The Jablochkoff
candles were also subsequently used in the electric lamps on the Thames
Embankment. The principle of the contrivance will be understood from
fig. 280_f_. Two carbons, C and D, are placed parallel at a little
distance apart, and the space between them is filled up with plaster of
Paris, kaolin, or some similar material, through which the current will
not pass, but which burns, fuses, volatilises, or crumbles away by the
heat produced by the passage of the current between the two carbons.
These carbons are, of course, fixed in insulated holders, and to start
the candle a small tip of carbon paste is made to connect the carbons at
the top. The Jablochkoff candles must be used with currents rapidly
alternating in direction. The reason for this is, that otherwise one of
the carbons (the positive one) would be consumed quicker than the other,
and that would cause the distance between them to increase, until it
became so great that the current would cease to pass, and the light
would go out. In order to obtain such alternating currents with the
Gramme machine, a special apparatus had to be devised to change its
direct into alternately reversed currents; but, dynamos intended to
supply electric lights are now made without commutators, and they supply
rapidly succeeding currents in opposite directions. In certain types of
dynamos, again, the armature coils are stationary, and it is the field
magnets that are made to revolve, and in these cases, not even a sliding
contact is required, but the end of the armature coils are directly and
permanently connected with the main circuit. But as these dynamos are
self-exciting, the electricity induced in a few of the armature coils is
collected apart from the main circuit, and passed through the
electro-magnets of the machine itself, after the alternate currents
have, by means of a commutator, been converted into one direct
continuous current.
[Illustration:
FIG. 280_g_.—_Electric Lamp._
]
The arc electric light, as used for the illumination of streets and
public places, is too intense and concentrated to be pleasant to the
eye, and therefore it has been found necessary to surround it by globes
of enamelled glass, or of porcelain, or of ground glass, or of frosted
glass. By these expedients for diffusing and softening the light, it is
rendered much more acceptable, but this advantage is gained at the cost
of a considerable loss of the whole illuminating power, a loss which is,
probably, never less than 10 per cent., but is usually much greater. The
globes used in Paris, with the Jablochkoff candles, were of enamelled
glass, and the apparatus was arranged, as shown in Fig. 280_g_, where it
is partly represented in section, and with a part of the globe broken
off, in order to show one of the candles placed in the holder which
connects it with the circuit. In each lamp several candles were mounted,
in some cases four; but the lamps in the Place de l’Opéra held twelve.
At first there were mechanical arrangements, automatic and otherwise, by
which, when the candle was burned down the current could be turned on to
another. But M. Jablochkoff afterwards discovered that there was really
no need for such a mechanism. For when the whole of the candles are
simultaneously and equally connected with the circuit conductors, it is
found that one of them will more easily transmit the current than any
one of the rest, and when that particular one has once been lighted by
the heat developed, the current will pass almost entirely through the
arc, any loss through the connecting strip of carbon, at the tops of the
other candles, being quite insignificant. When the first of the candles
has burnt down completely, until the insulating porcelain holder
separates the carbons, the current will at once re-establish itself at
the top of one of the remaining carbons, and so on, while one is left.
The arc electric light has not been brought to its present position
without the expenditure of much care and ingenuity in the preparation of
the carbons used for its production. When Davy first produced the
voltaic arc, the electrodes he used were simply sticks of charcoal.
These were very quickly consumed, and a more durable form of carbon was
sought for. This was found by Foucault, who made use of rods sawn out of
the carbonaceous residue left in the retorts in the process of making
coal-gas. This substance was, however, by no means uniform or
sufficiently pure, and the light obtained was consequently unsteady.
Many experiments were made in preparing special carbons. Pounded coke,
coke and charcoal, were mixed with syrup or tar into a paste, which was
moulded and compressed, and then the sticks were kept in covered vessels
at a high temperature for many hours. Acids were used for purification,
and also alkalis, to remove silica. At the present time there are
several manufacturers of electric light carbons who carry on extensive
operations by processes which probably are very similar one to another,
and which may well be represented by M. Carré’s, whose carbons have the
highest reputation. M. Carré prefers a mixture of powdered coke,
calcined lampblack, and a syrup made of sugar and gum. The whole is well
mixed and incorporated, water being added from time to time to make up
for loss by evaporation, and to give the paste the proper degree of
consistence. The paste is then subjected to compression, by which it is
forced through draw-holes, and the carbons, having been piled up in
covered crucibles, are exposed for a certain time to a high temperature.
As a practical illuminant for lighthouses, the arc electric light came
into use many years ago (1862) as we have already seen. This was when
the generator of the current was the magneto-electro machine; but, now,
when this generator has developed into the modern dynamo, the cost of
the electric supply has been enormously reduced, so that, power for
power, electric lights may be worked at half the former cost, and with
greater convenience and certainty. Light for light, electrical
illumination is said to be far cheaper than gas. Again, the arc electric
light has properties which have caused it to be employed, not only in
every important lighthouse in England, France, Russia, America, and
elsewhere, but most ships of war are provided with means of projecting a
beam of electric light in any direction, in order that the presence of
torpedo boats, etc., may be discovered at night, or harbours entered and
signals made under circumstances when such operations would be otherwise
impossible. It was by the use of the electric light that, in 1886, one
of the Peninsular and Oriental Company’s steamers passed safely through
the Suez Canal, at night, and the experiment was so satisfactory, that
the canal authorities placed beacons and light-buoys to guide such
vessels, as, being provided with electric apparatus, were enabled to
hold their proper course between its banks. The use of projected beams
for watching the movements of enemies, and for signalling to great
distances in time of war, has been recognized by all the great military
powers. The advantage of the electrical light in some mines, in
subterranean and submarine operations and generally, in work that has to
be carried on at night by large bodies of men, is constantly finding
illustration. Few readers are unacquainted with the brilliant effect of
the arc lamps in exhibitions, parks, &c.; at out of door fêtes, or
applied to the illumination of fountains, such as those at the Paris
Exhibition of 1889.
The arc lamps are used in series; that is, where there are a certain
number of lamps to be supplied, the same electrical current circulates
through the whole of them, and this, of course, must have force enough
to overcome the resistance of the whole circuit. Thus, at each lamp, the
intensity of the illumination must necessarily be very great. A solution
was long sought to the problem of so dividing the current energy, that
it might be made to produce lights, of moderate intensity, at a greater
number of points. When Mr. Edison, shortly after having invented the
phonograph, announced that he had solved the problem of the electric
light division, there was a great panic amongst the holders of shares in
gas companies, and a heavy fall in this kind of stock immediately
occurred. As it turned out, the alarm was unnecessary, for gas was not
to be superseded, immediately and definitely, by electricity.
Nevertheless, it is by virtue of the principle that was contained in
Edison’s invention, that electric lighting has assumed the wide-spread
importance it has at the present day, and that it is now actually
ousting gas as an illuminant in the business and domestic premises of
our large towns, and in theatres, libraries, and other places of resort.
The principle which has brought about this great development of electric
illumination is that shown in a simple form in Fig. 261. It appears,
however, that as early as 1841, a platinum wire, made incandescent with
a battery current, was proposed as a source of light, and in 1845,
carbon was used in the form of slender rods, by King, and also by J. W.
Starr, in the United States. Both inventors inclosed their carbons in
glass tubes, from which the air was exhausted, so that the carbon might
not burn away. In the following year, Greener and Staite turned their
attention to lamps of this kind, and, again, in 1849, Petrie worked on
the same subject. After that, the problem ceased to engage attention,
until, in 1873, a Russian man of science, named Lodyguine, took the
matter up and patented a carbon incandescent lamp, which did not,
however, prove a practical success, and although the idea was worked out
in various ways by Konn, Reynier, Trouvé, and others, the apparatus they
designed was, in every case, lacking in simplicity, and certainty of
action. The Edison incandescent lamp, the announcement of the discovery
of which so fluttered the gas companies, about 1878, was a reversion to
the plan of an incandescent metallic wire. This wire was made of an
alloy of platinum and iridium, which was adopted by Edison on account of
the very high temperature required for its fusion. And in order to
prevent the temperature from quite reaching that point, the wire was
arranged in a spiral within which was a rod of metal that, by its
dilatation with a certain temperature, caused a contact to be made which
diverted part of the current through a shorter circuit, and thus lowered
the temperature of the spiral to within the assigned limits. But the
advantages presented by carbon over metallic conductors led Edison to
attempt the formation of filaments by charring first slips of paper,
afterwards slips of bamboo. About the same time Mr. J. W. Swan, of
Newcastle-on-Tyne, was experimenting in the same direction, and, in the
latter part of the year 1880, he exhibited the first incandescent lamps
shown in England. Swan’s carbon filaments were prepared from cotton
threads which had previously been steeped in dilute sulphuric acid,
washed, and passed through draw holes to give them an uniform section.
They are thus made perfectly homogeneous throughout, and, after having
been wound on pieces of earthenware to the required shape, they are
carbonized by packing in powdered charcoal and heating. These filaments
are very thin, but solid and elastic. The arrangement of the lamp (see
Fig. 280_h_) is extremely simple: the filament of carbon bent into a
horse-shoe form, or turned so as to form a loop, is inclosed in a glass
bulb of a globular or egg shape, about two inches in diameter. The
extremities of the filament are connected in an ingenious manner to two
platinum wires that pass outward through the glass into which they are
fused, and terminate either in binding screws or in two small loops. The
bulb is exhausted first by an ordinary air-pump, and then by a Sprengel
mercurial pump, the current of electricity being sent through the
filament during the last stages of the process, and finally the bulb is
hermetically sealed. The light yielded by these lamps is mild and
steady, and its intensity depends on the electric current sent through
them; but this may, it is said, be carried as high as to make the light
equal to that of twenty candles. Each horse power of force expended on
the dynamo suffices to maintain ten of these lamps. At the Exhibition of
Electrical Apparatus at Paris in 1881, the Swan lamp received the gold
medal as being the best system in its class. The Swan and the Edison
patents are now worked together by one Company, and the productions of
this Company are very largely used, although there are several more or
less modified systems of glow lamps prepared by other manufacturers.
[Illustration:
FIG. 280_h_.
]
The great advantages offered by electric glow lamps over gas-lights
caused them to be speedily adopted by the most enterprising managers of
theatres and places of amusement. Mr. D’Oyly Carte had the Savoy
Theatre, in London, completely fitted up with these lamps in 1881. The
light was soft and agreeable, it did away with the risks of fire both
for the audience and the performers: for the footlights and scene-lights
were also electric glow lamps, and the coolness of the house and greater
purity of the air were at once appreciated. Several other London
theatres have since adopted the incandescent electric lamps, and it is
obvious that the system will become universal. In all ocean-going
passenger steamers, electric lighting of the saloons and cabins is now
the rule. No mode of illumination so readily adapts itself to the
production of artistic and decorative effects as the glow lamps: for the
covering glasses may be tinted of any required shade, and the lights may
be placed in any position. Small glow lamps are occasionally used as
personal adornments, when placed, for instance, as part of a lady’s
head-dress amidst diamonds, a novel effect of great brilliancy is
produced. It need hardly be said that in this application the wearer is
not required to carry a dynamo about with her, for the electricity is
supplied in a manner much more convenient for this purpose by a device
presently to be described. For several years electric incandescent
lamps, supplied by the like means, have been in action every night in
the carriages of the trains running between London and Brighton, and
more recently the Company have had electric reading lamps of five
candle-power fitted up in the carriages of the main line trains. They
are placed at the backs of the seats just above the passengers’ head.
When anyone wishes to make use of one of these lamps, he places a penny
in a slot, and then, on pressing a knob, the light appears, and at the
end of half an hour it is automatically extinguished; but, of course, it
can again be made to appear by another penny dropped in the slot, and so
on every half-hour as long as may be required.
To maintain the electric light (whether arc or incandescent) quite
steady, the greatest uniformity in the speed of the dynamo is essential;
and if the prime mover by which it is worked, whether steam-engine,
gas-engine, water-wheel, or turbine, is not perfectly regular in its
action, the lights will fluctuate in brightness, and thus produce an
effect which is very unpleasant. This is entirely obviated by the
adjunct we have now to describe, which not only is most efficient as a
regulator, but is, moreover, of still more importance by also providing
the means of storing up the electrical energy in a portable form. The
reader will have understood that in a voltaic cell the production of an
electric current is the concomitant of a chemical union of substances
within the cell (p. 493). Now, in the experiment shown in Fig. 263 (p.
498), it is the reverse of combination—namely, the decomposition of the
water that is supposed to be effected under the influence of the current
from a galvanic battery, and the poles are so connected that the
direction of the current in the liquid while the decomposition is
proceeding is from the wire in the O tube to that in the H tube. If the
experiment be interrupted by removing the battery, and then putting a
galvanometer (Fig. 258) in its place, the galvanometer will immediately
indicate a current passing through the apparatus in a direction the
_reverse_ of the former one—that is, in the liquid it goes from H to O,
and the volumes of the gases will slowly diminish while water is
reproduced by imperceptible and gradual re-combination. Batteries can be
made by joining up a series of arrangements like Fig. 263, consisting of
nothing but strips of platinum surrounded by hydrogen and oxygen gases
and the intervening acidified water. Analogous results are obtainable by
cells containing other compounds with suitable metallic poles, for when
decomposition has been effected through a series of such cells by a
sufficiently powerful current from a _primary_ battery, the series of
cells will constitute, on removal of the primary battery, a _secondary_
battery, for when the terminals of this are joined, the current will
flow in the reversed direction while the separated parts of the original
compounds are re-combining within the cells. These _secondary_ batteries
are called also _polarisation_ batteries. A form of secondary battery
was contrived some years ago (1859) by M. Gaston Planté, in which the
current of the primary battery was made to act on plates of lead
immersed in dilute sulphuric acid. The effect was to coat one of the
lead plates of each pair with lead oxide; and in the action of the
secondary battery this was reversed, and the plates gradually returned
to their original condition, when, of course, the current ceased. Some
improvements were made in the Planté battery by Faure, who coated one of
a pair of very thin lead plates at once with a film of red oxide of
lead, and used a layer of felt to separate it from the other plate. Such
arrangements have been called “accumulators”; another term applied to
them is “storage batteries”; but it is not to be supposed that in them
electricity is stored or, so to speak, bottled up. They consist merely
of such an arrangement of materials as that when a current (_direct_,
not alternating) from a dynamo is passing, certain substances are placed
in a position of chemical separation in such a manner that in
re-combining an equable current of electricity is produced in the
conductor externally uniting them. We need not notice some slight
modifications of the Faure cells that have been lately introduced, as no
new principle is involved. The light of incandescent lamps worked by the
Faure accumulator is perfectly free from the fluctuations which may
usually be noticed when the lamps are directly connected with the dynamo
only. Even if the engine should stop altogether, the light may be
maintained for hours. The accumulator has also the advantage of giving
out the electric energy that may have been imparted to it days before;
so that when a house is fitted up with an independent electric light
installation, there is no necessity for running the dynamo all the time
the lamps are in use, as two or three days weekly may suffice to charge
all the accumulators. Then there is the portability of the accumulator,
which permits electrical energy to be made use of in situations where
dynamos and prime movers would be impossible. It is said that a large
Faure cell weighing about 140 lbs. can receive and give out energy equal
to one horse power for one hour. In the arrangement for the reading
lamps in railway carriages referred to above, accumulators are placed
under the seats; and it need hardly be said that when the electric light
has been seen in a _coiffure_, a small Faure cell concealed about the
wearer’s person has supplied the current. A very interesting and useful
application of the accumulator is the portable electric light lamp for
miners made by the Edison-Swan Company. It is simply an incandescent
lamp protected by a strong glass cover attached to the side of a
cylindrical case containing a four-celled accumulator. This lamp is
provided with an ingenious contrivance by which the circuit would be
interrupted, if by accident the outer glass cover of the lamp were
broken. Let us now see what another new development of the applications
of electricity gains by the use of accumulators by turning our attention
to the _electro-motor_.
At the Vienna Exhibition of 1873, the Gramme Company showed two of their
machines, and it is said that when one of these machines was at rest, a
workman connected the ends of two covered copper wires with the other
machine, thinking that these were placed to carry the current from that
machine when in movement. Everybody was surprised when, without any
power from the machinery, the ring was soon in rapid rotation. These
wires were in fact joined up to the other Gramme machine which was
already in action, and it was the current from this that set the former
in motion. There is no reason why this story should not be perfectly
true, although there are good reasons for believing that the
_electro-motor_ was the result of no such accidental circumstance. The
attractions and repulsions between the poles of electro-magnets was soon
seen to supply an available source of motive power, and the subject has
been already mentioned on page 518. Professor Jacobi, of St. Petersburg,
seems to have been the first who constructed an electro-magnetic engine,
the exciting power being the current supplied by a voltaic battery. This
was in 1834, and in a few years afterwards the Professor applied his
engine to a small paddle-wheel boat, 28 feet long, which was
electrically propelled for several days, but at a slow speed. The engine
in this case was virtually a magneto-electric machine worked backwards,
that is, instead of applying power to turn the machine and so produce a
current of electricity, the current was supplied by the battery and
produced power. In 1850, an electro-motor of five horse power was shown
by an American, Mr. Page, the principle of which may be illustrated by
supposing a reversal of the action represented in Fig. 271, thus: if,
instead of producing currents by moving the magnet, C, in and out of the
coil, A B, we substitute a battery for D, we can, by alternating the
direction of the current through the coil, cause a reciprocating motion
of the magnet, C, and this again may be described as a magneto-electric
machine worked backward. It was soon recognized that no practical
electro-motor was adequate to the production of such high powers as the
steam engine supplies, and that the cost must necessarily many times
exceed that of steam power. But certain advantages, nevertheless,
pertained to the electro-motor in certain positions, as instance in
safety, and where a small force only was occasionally required. Now,
when the Gramme machine was invented to supply currents of electricity
under conditions much more favourable than the magneto-electric machines
it superseded, and at a cost vastly less than that of any voltaic
battery, it is highly improbable that the relation of the new current
generator to the production of electro-motive power would long be
overlooked.
The electro-motor may, therefore, be considered simply as a dynamo
worked backward, and almost any form of dynamo may in this way be used
as an electro-motor, that is, a current being supplied either from a
battery or from a dynamo, the motor converts the electrical energy into
mechanical energy. Any dynamo that supplies a direct and continuous
current can thus be used; but there are certain conditions which make it
desirable to somewhat modify the proportions and arrangement of the
several parts when the machine is for motor purposes.
In general, any source of current may be used, but in the applications
of the electro-motor there are chiefly two methods in practice of
supplying the current. The one takes the current from a dynamo in
motion, the other from an accumulator which has previously been
“charged” by a dynamo.