Part 71

It would be impossible within our limits to even briefly describe the great number of improvements and modifications of Bell’s system that were devised by various persons soon after the invention was brought out, and many additional complications were introduced into some of the arrangements. Advantage was also taken to a greater or less extent of another principle affecting the strength of electric currents, to which we have now to call the reader’s attention, and to exemplify by one of the simplest instruments, leaving detailed accounts of the various forms in which it has been applied to be found in special treatises. The reader should first turn back to page 400, where he will see an expression of the strength of a battery current. It will be observed that the current may be increased or diminished by diminishing or increasing R, the external resistance, without changing the other terms. Now M. Du Moncel discovered, as far back as 1856, that an increase of pressure between two conductors in contact, and conveying a current, caused a diminution of the electrical resistance, and this discovery was utilized for telephonic purposes by Mr. Edison in his invention of the carbon transmitter (1876). In this there is no magnet, and a stretched membrane may take the place of the metallic plate, although a circle of photographers’ ferro-type plate gives better results. A pad of india-rubber, cork, or other material is fixed on the plate, and rests upon a carbon disc, which again is in contact with a metallic conductor. Between the latter and the carbon the current from a constant battery passes. When the plate is thrown into vibration by speaking into the mouth-piece, the variations of pressure conveyed to the carbon cause variations in the resistance of its electrical contact, and thus a series of undulations are produced in the current, and these affect the electro-magnet of a Bell receiving instrument in the circuit as before, so that the sounds are reproduced. It is now time to say a word about the share in the invention of the speaking telephone which has been claimed by Mr. Elisha Gray, also of the United States, who, at the time Mr. Bell applied for the patent for his instruments, produced drawings and descriptions of a plan he had devised for transmitting speech by undulating electrical currents, and it has been admitted that the plan he had conceived was perfect in principle. He proposed to use a battery current, and his receiving instrument was nearly the same as Bell’s. The undulations of the current were also determined, as in Edison’s telephone, by changes in the external resistance, but this was effected in a different, though equally simple manner. To a membrane stretched across the lower end of a short wide tube that formed the mouth-piece of the transmitter, and was placed vertically, was attached a piece of platinum wire, conveying the current and dipping into a liquid of moderate conductivity, but not quite touching another platinum electrode fixed at the bottom of the vessel containing the liquid. The space of liquid traversed by the current being thus varied by the oscillations of the membrane, the resulting variations of the resistance produced the requisite undulations in the intensity of the current. Both Mr. Bell and Mr. Gray applied for patents on the 14th February, 1876, but the American Patent Office recognized the claim of the former as prior.

Du Moncel’s observation was applied by Mr. Hughes in the construction of an instrument, which he named the _microphone_. This was in the same year that Edison had brought out his carbon telephone, and a certain similarity, resulting from the identity of the principle employed, led to an acrimonious controversy on what were supposed to be rival claims. But the microphone differs so much in arrangement and performance from the other instrument as to constitute a distinct invention. The instrument, if it may be so called, is simplicity itself, in the form represented in Fig. 302_e_, which is one of the most sensitive. There, C and C´ are two small blocks of carbon, fixed on a small upright piece of wood. Two cup shaped cavities are hollowed out in the carbon blocks, and these serve to hold loosely, in a nearly vertical position, a small rod of gas retort carbon pointed at the ends. This rod is only about one inch in length, and the lower end merely rests on the bottom of the cup in C´, while the other is capable of moving about in the upper cavity, the vertical position being nearly maintained in a state of unstable equilibrium. The carbons are in the circuit of a voltaic cell or small battery, B, in the line through a Bell receiving instrument, which may be at a distance. When the microphone is to be used, it is placed on a table with a cushion or several folds of wadding beneath its base. If the receiver be applied to the ear of a listener, he will distinctly hear every word pronounced by one speaking near the microphone, even in a low tone; but a loud voice may be heard when the speaker is 20 or 30 feet from the instrument. The minutest vibrations conveyed to the stand are perceived at the receiver as loud noises. The tread of a fly walking over the board, S, is heard like the tramp of a horse, and the ticks of a watch are audible in the receiver when the ear is several inches away from it. The slight touch of a feather on the stand is distinctly audible, and a current of air impinging upon it is reproduced as the noise of a stream of water. The microphone is, in fact, the most sensitive detector of vibrations that is known, and its employment as a transmitter has brought the telephone to its present perfection. It has been constructed in an endless variety of forms, according to the purposes for which it is intended, and its simplicity is as wonderful as its extreme sensitiveness. We will further illustrate these qualities by an experiment of Mr. Willoughby Smith’s on the same principle. Instead of the two carbon blocks, he laid on the table, in parallel positions, two small rat tail files, and completed the circuit by a third file, laid across the others at right angles. This arrangement constituted so sensitive a transmitter that the listener at the distant Bell receiver could hear even the faint sound of the speaker’s breathing. Even three common pins, similarly crossed, make an effective transmitter. The feebleness of the variations in the current requisite to make the Bell receiver produce sounds is extraordinary, and a very weak battery current is sufficient, even under the circumstances of ordinary practical use. Still more remarkable is the fact that in favourable conditions the microphone is capable of transmitting sound without any battery at all, but merely with connections to earth, when the ticking of a watch placed upon the stand has been distinctly heard at the distance of nearly one-third of a mile, and speech, also, has been transmitted with unusual distinctness with the battery left out and merely a few drops of water placed at the carbon contacts; indeed, it is said that, even without the water, the voice may be heard. This effect has been attributed to the carbons and water forming a battery themselves, and in the latter to the moisture of the speaker’s breath supplying the fluid element. But, again, the microphone will not only transmit speech, but, under certain arrangements, it will reproduce it (when one of the carbon electrodes is attached to a membrane), although the result is less distinct than with the Bell receiver. It is, however, not so easy to explain how mere variations of current intensity can produce the effect where there can be no magnetic attractions and repulsions. We must, no doubt, look for the cause in some other property of electric currents. The transmitters used in various lines of telephonic communication, erected by the Post Office or by companies in Great Britain, are generally applications of the principle of the microphone, and not of that of either Mr. Bell’s or Mr. Edison’s original instrument. But more recently, Mr. Edison has most ingeniously adapted variations of sliding friction, as modified by the action of the undulatory current on a liquid electrolyte between the sliding surfaces to the production of a loud speaking telephonic receiver—that is, one by which the sounds are made audible to a large assembly. From this instrument, the notes of a cornet-à-piston, played in Brighton, have been distinctly heard throughout a large hall in London.

Another curious transmitter is formed of a fine jet of water traversed by an electric current. Acoustic vibrations are easily set up in the jet, and these modify its conductivity so as to produce corresponding undulations of current intensity.

It would take long to point out the many scientific applications of so sensitive an instrument as the microphone with its Bell receiver. As a medium for conveying speech to a distance, whether for purposes of peace or war, its use is sufficiently obvious. Some curiosities of musical transmission have been noticed, and such experiments are repeated from time to time with increasing success. It has been applied to many purposes in surgery and medicine. In many cases of deafness it has made conversation easy. Even the passage of the molecules of gases, when diffusing through porous partitions, Mr. Chandler Roberts has by its means made audible. The distances to which speech can now be transmitted are considerable, as conversations have been carried on by persons nearly 300 miles apart.

LIGHTHOUSES.

Who does not regard with interest the lighthouses which at night throw their friendly beams across the sea, to guide the mariner in his course, and warn him of perils from sunken rock or treacherous shoal? The modern lighthouse, with its beautiful appliances, is entirely the result of the applied science of our age; and it affords a fine example of the manner in which experiments, carried on to determine natural laws apparently of an abstract character and without any obvious direct utility, give rise to inventions of the highest importance and most extended usefulness. The lofty structures which were erected near certain ancient harbours, and of which the Pharos of Alexandria is the most memorable example, burned on their summits open fires of wood; and whatever beacons existed from that time down to the end of last century were merely blazing fires of wood or coal. The lighthouses of the South Foreland, which were established in 1634, displayed coal fires until 1790, and the lighthouses in the Isle of Man were first illuminated with oil only in 1816. Down to the beginning of the present century, therefore, the modern lighthouses showed no improvement on the ancient plan. Even the Tour de Cordouan, at the mouth of the Garonne river, which was completed in 1610, and is one of the most famous of modern lighthouses, from its great height (200 ft.), and the care which has always been given to render it efficient, showed down to 1780 merely a fire of billets of wood, the upward loss of the light being diminished by a rude reflector in the form of an inverted cone. In the improved means of obtaining artificial light, and in the admirable optical apparatus by which that light is utilized, we find the vast superiority of modern lighthouses. But these are sometimes erected on isolated, and almost submerged, rocks, exposed to the fury of the waves. The difficulties which have to be overcome in their construction cause some lighthouse towers to rank among the best specimens of engineering skill. We may, therefore, consider under the present head—the towers; the sources of light; the optical apparatus and its accessories.

One of the best-known lighthouses on the English coast is that on the Eddystone Rock, about 14 miles S.S.W. from Plymouth. The structure which now[9] stands upon this rock was the work of Smeaton, and was completed in 1759. The stones forming the lower courses of this tower, which is represented in Fig. 303, half in section and half in elevation, are dovetailed into the rock itself and into each other. The masonry is carried up in a solid mass for about 12 ft., the stone used being granite, which also constitutes the whole of the exterior masonry. The four upper apartments are formed with arched roofs, the side-thrust of which is counteracted by iron chains surrounding the tower. These chains, which are bedded in lead, were placed in their positions while hot, and by their contraction bound the structure together with great force. The masonry of the tower is 68 ft. high, and this is surmounted by the light-room, the total height from the lowest course of stonework to the gilt ball at the top being 94 ft., or nearly half that of the London Monument. The diameter at the base is 26 ft., and that at the top 15 ft. The light-room is of an octagonal shape, and is made of iron framework, glazed with thick plate glass. Below this are two store-rooms, a kitchen, and a bed-room. The Eddystone has now breasted the storms of more than a hundred years, and it remains as firm as the rock it is built on. Fig. 304 is a picture of this noble lighthouse, with the British fleet passing close to it, during a furious gale on the 22nd of October, 1859, or exactly a century after the completion of the structure. The incident of the man in the water, which occupies the foreground, is not an imaginary one, for it is recorded that the _Trafalgar_ stopped in the midst of the storm to pick up a man who had fallen overboard. For eighteen hours the ships encountered the fury of the tempest, keeping out at sea in open order throughout the night. They wore in at dawn, came up the Channel in line of battle, steamed into Portland, and took up their anchorage without the loss of a sail, a spar, or a rope-yarn.

Footnote 9:

Smeaton’s tower proving unsafe, has since been taken down and replaced, in 1882, by one from Mr. Douglass’ design.

The lighthouse tower on the Bell Rock is 100 ft. high, 42 ft. in diameter at the base, and 15 ft. at the top. The Inchcape Rock, on which it is placed, is the scene of Southey’s ballad of “Ralph the Rover,” and the lighthouse here is one of the most serviceable on the Scottish coast, for the dangerous spot on which it is placed lies in the direct track of all vessels entering the Firth of Tay from the German Ocean. The rock is submerged at spring tides to the depth of 12 ft. The tower bears a close resemblance in shape to that of the Eddystone: it is circular and faced with massive blocks of granite. The lower part, to the height of 30 ft., is solid, and the door is reached by a bronze ladder. The building contains five apartments, and a cistern for storing fresh water for the use of the keepers, who have sometimes to remain in their solitary situation for six or eight weeks together, the weather preventing the possibility of any communication with the shore.

Still loftier than the tower on the Bell Rock is that which rises in the midst of the Skerryvore Reef, 12 miles from Tyree, a small island off the coast of Argyleshire. This building may be taken as a typical specimen of a detached lighthouse, and the difficulties overcome in its construction attest the skill of the engineer, Mr. Alan Stevenson, who has written a highly interesting account of the work. The rocks here are of _gneiss_, an extremely hard formation, and their surfaces are worn as smooth as glass by the action of the water. On one of a numerous series of these small islets, where only a narrow strip of rock, a few feet wide, remains above the surface at high water, and this divided by rugged lumps into narrow gullies, through which the sea constantly rushes, the lighthouse is built. The work was commenced in 1838 by the erection of a temporary wooden barrack on piles at a little distance from the site chosen for the foundation. In a gale during the winter the whole of this structure was swept away in one night. Another, more strongly secured, was built the following summer, and in this Mr. Stevenson and his men remained sometimes for fourteen days together, the weather preventing any passage to or from the shore: here the men were sometimes awakened from their hard-earned repose by the water pouring over the roof, and by its rushing through the crevices, while the erection swayed and reeled on its supports. Mr. Stevenson relates that one night the men became so alarmed for the stability of their shelter that some descended, and sought in cold and darkness a firmer footing on the rocks. Two summers were occupied in cutting the foundations, and the blasting of the rock in so narrow a space was an operation attended with no little danger. A small harbour had to be formed at the rocks for the vessels bringing the ready-prepared stones of the building from the quarries, where also piers were built expressly for the shipment of the materials. In designing his tower, the engineer preferred to oppose the force of the waves by the weight of his structure, rather than to rely on dovetailed or joggled-jointed stones. Measurements were made of the force of the waves, which at Skerryvore was sometimes equivalent to a pressure of 4,335 lbs. on the square foot; and calculations based on these measurements showed that the mere weight of the superstructure would amply suffice to keep the stones immovable. Nearly 59,000 cubic feet of stone were used, or about five times the quantity contained in the Eddystone Lighthouse, and the total cost of the building was £87,000.

The use of iron, as a building material advantageously replacing stone, has extended to lighthouses, and many have been constructed entirely of cast and wrought iron, or partly of iron and partly of gun-metal, which is not readily acted on by the sea spray. Such lighthouses are cheap, easily and quickly erected, strong enough to bear shocks and vibrations, and proof against fire, lightning, and earthquakes. The lighthouse on Morant Point, Jamaica, is made of iron, cast in England; and it was erected in a few months at a cost of one-third of that of a stone tower of the same altitude. Its height is 105 ft., and the shaft is formed of iron plates in segments of 10 ft. high, which are bolted together at their flanges. At Gibbs Hill, Bermuda, is a lighthouse 130 ft. high, constructed in the same manner.

So inefficient, inconvenient, and uncertain were the lamps or other means of artificial illumination known up to nearly the beginning of the present century, that nothing better could be found for the Eddystone Lighthouse for forty years after its erection than tallow candles stuck in a hoop—a means of illumination which would scarcely now be tolerated even in a booth at a village fair. To M. Argand, a Frenchman, we are indebted for the first great improvement in lamps. The admirable invention which bears his name is, as everybody knows, an oil lamp with a tubular wick, which occupies the annular space between two metallic tubes, in such a manner that a current of air rises through the inner tube, and thus reaches the interior of the flame. This current, and the current which supplies the exterior, are increased by surrounding the flame with a tall glass chimney; and a contraction of the chimney, just above the flame, aids greatly in distributing the air, so as to insure the complete combustion of the oil. In the original lamp the supply of oil to the flame depended on the capillary attraction in the meshes of the wick. M. Carcel applied clockwork to continuously pump up the oil into the burner, so that, by overflowing, it was maintained at an invariable level. This arrangement added greatly to the intensity and steadiness of the light; and, on account of the uniformity of its flame, the Carcel lamp has been selected as a standard to which, in France, photometric determinations are referred.

The power of the Argand lamp, as employed in lighthouses, is greatly increased by the plan of employing several concentric wicks instead of one. Between these wicks there are, of course, open spaces, through which the air obtains access to the flame, and the current of air is made more rapid by the use of a very tall chimney. The large amount of heat produced by the combustion of so much oil in a small space is partly carried off by the excess of oil which is made to overflow the burner—about four times the quantity consumed being constantly pumped up into the burner for this purpose. Lighthouse lamps are made with two, three, and four wicks; and the oil is forced up in the burners either by clockwork or by the pressure of a piston loaded with a weight. The following table gives the sizes of the burners and the illuminating powers of the lamps:

┌──────┬──────┬────────┬─────────┐ │Order │Number│Diameter│Intensity│ │ of │ of │ in │of Light │ │Light.│Wicks.│inches. │in Carcel│ │ │ │ │ Lamps. │ ├──────┼──────┼────────┼─────────┤ │ 1 │ 4 │3½ │ 23│ │ 2 │ 3 │2–15/16 │ 15│ │ 3 │ 2 │1¾ │ 5│ └──────┴──────┴────────┴─────────┘

The quantity of oil consumed in these lamps is less than that proportional to the increase of the light:—for example, although the four-wick lamp gives twenty-three times the light of the simple Argand, it only consumes nineteen times the quantity of oil. The oil used in these lamps is colza; but experiments have been made with a view of introducing petroleum, which has the advantages, not only of being cheaper and uncongealable by cold, but of giving a whiter and more brilliant light. Hitherto, however, this substance has answered only with lamps of one wick.

Coal-gas has been applied to the illumination of lighthouses, and as it gives a light of great brilliancy and steadiness, when consumed in proper burners, it has certain advantages over oil lamps, which have caused it to be employed in situations where a supply can be readily obtained. The light produced by lime, ignited by the combustion of coal-gas or hydrogen mixed with oxygen, has also been suggested; but this plan is not without risk of interruptions and of dangerous accidents, and it has been considered inadvisable to entrust the apparatus to the persons who commonly take charge of lighthouses.

The electric light has been very successfully applied in certain lighthouses, since the mode of producing steady currents by magneto-electric[10] machines has come into use. The lighthouses at the South Foreland have been thus illuminated by a machine constructed by Mr. Holmes in 1862. A very powerful electric light is exhibited from the lighthouse on Cape Grisnez; and the adoption of this source of light has been extending, as it is far more intense than any other artificial light, and can be sent in more concentrated beams across the sea, on account of its being emitted from a space which is practically a point. These circumstances cause the beams of the electric light to possess greater power of penetrating the atmosphere than those from any other source.

Footnote 10:

Now superseded by the dynamo-electric machines.