Part 11
The conducting power of the earth is now employed by all electric telegraph companies for one-half of every circuit. Thus, whether a communication be sent from London to Liverpool, to Edinburgh, Paris, or Brussels, the moist earth serves to complete one-half of the communication. In the telegraphic circuit between London and Liverpool, for example, the insulated wire is connected at each end with the earth by being soldered to a copper plate, which is buried a few feet underground, so as to insure its being always surrounded with moisture. To improve the connection of this plate with the earth, it is customary to bury with it a quantity of sulphate of copper, the solution of which surrounds the earth-plate with a better conducting liquid than water, and thus extends the connecting surface. The gas pipes or water pipes are sometimes employed for the attachment of the wires instead of an earth-plate, but the latter is generally preferred.
In arranging a telegraphic circuit, the voltaic batteries and the instruments are introduced at breaks in the telegraph wire. The course of the electric current is from the copper end of the battery through the transmitting instrument, then along the wire to the receiving instrument; from that it passes to the earth and is thus returned to the transmitting station, where it completes the circuit by being conducted from the earth-plate to the zinc end of the voltaic battery. The arrangement for completing the circuit will be more clearly understood by reference to the accompanying diagram.
The wire from _C_, which is the copper pole of the voltaic battery, is connected with the instrument _A_; the electric current is then transmitted along the wire _D_ to the receiving instrument _B_; thence it is transferred to the earth-plate _E_, passes through the earth to the corresponding plate _E´_, which is connected with _Z_, the zinc pole of the battery. When a communication is returned from _B_ to _A_, a similar arrangement is made; the wires connected with the instruments being so arranged as to bring into action a voltaic battery at _B_, and to throw out of circuit the one at _A_; for the connection with the battery is only made when the transmitting instrument is worked.
Since all the electric telegraphs in different parts of the world are connected with the earth, as one portion of the circuit, it might be supposed that the various currents would mingle, and occasion a confusion of messages; but it must be borne in mind that no electric current is formed until a communication be made from one pole of a voltaic battery to the other, and as such communication can only be completed through the insulated wire, the earth-currents cannot mingle, but each one passes to the proper terminus of its respective battery. The accompanying diagram and explanation may serve to remove the difficulty of understanding why the two circuits are maintained quite distinct.
The letters _A_ _B_ represent the wires making communications between the batteries _D_ and _E_, and the telegraph instruments _I_ _O_ at the receiving station. The electricity from the copper end of the battery _D_ would be conducted along _A_ through the instrument _I_, and by the wire _K_ to the earth-plate _H_. It would be then transmitted through the earth on its return to the battery, in the direction of the arrows, to the other earth-plate _G_, and thence it would find its way to the zinc pole of the battery _D_, and complete the circuit. In the same manner, the electric current from the copper end of the battery _E_ would be transmitted through the wire _B_, and would complete its current also by means of the earth-plates _G_ _H_, and would traverse the course indicated by the arrows, and return to the zinc end of _E_. Though both electric currents traverse the same wire from the instruments _I_ _O_ to the earth-plate _H_, and are thence transmitted through the earth to a single plate, _G_, at the transmitting station, there is no mingling of currents, the electric current of each battery being kept as distinct as if separate wires were used both for the transmitted and the return current. It would, indeed, be as impossible for the separate currents transmitted from the two batteries to be mingled together, as it would be for the written contents of two letters enclosed in the same mail-bag to intermix.[8]
The length of telegraph lines at present laid down by the several telegraph companies in Great Britain, exceeds 10,000 miles. To complete those lines required 40,000 miles of wire, and there are 3,000 persons engaged in the transmission of telegraphic intelligence.
In North America there is a direct communication from New York to New Orleans, a distance of 2,000 miles, through the whole length of which wires messages can be transmitted without any break. Wires have also been suspended on lofty posts across the Indian Peninsula, where no railways have been yet laid down. Lines of insulated wire, partly submerged in the sea, partly buried underground, and partly suspended on posts in the air, place London and Vienna in direct communication; and other telegraph lines are in the course of construction, which will unite London with Africa: and a complete net-work of telegraph wires is spreading over the face of Europe.
It will not be long before this system of communication is connected with a similar one in America. The failure of the cable already laid down has confirmed the opinion of the author, expressed in papers read at meetings of the British Association for the Advancement of Science, and in his work on Electricity, that the conducting wire should be sufficiently strong to be self-protective, without requiring an external coating of iron wire rope. A conducting copper wire, a quarter of an inch in diameter, covered with gutta percha and tarred hemp, would be more flexible and stronger than the combined cable; and it being a much better conductor of electricity, the rapidity of transmission would be greatly increased.
The effect of the establishment of competing telegraph companies in England has been to diminish the charge for transmitting messages, in some instances to one-fifth of the rate formerly demanded; and when further experience in the construction of telegraphic lines, and the adoption of more rapidly transmitting instruments, have facilitated and improved the means of communication, we may anticipate that correspondence by Electric Telegraph will in a great measure supersede the transmission of letters by post.
ELECTRO-MAGNETIC CLOCKS.
The invention of Electro-Magnetic Clocks closely followed the introduction of the electric telegraph; and Professor Wheatstone, to whom the world is principally indebted, in conjunction with Mr. Cooke, for the perfection and application of the needle telegraphic instrument, claims also to be the original inventor of Electro-Magnetic Clocks. His claim is, however, disputed by Mr. Bain, who asserts that he was the first who conceived the idea of applying the power of electro-magnets to the regulation and movements of clocks, and it must be admitted that he brought the invention into a working state.
In the first stage of the invention, the object attempted to be attained was to regulate several clocks, once an hour--or oftener, if required--so that they might all indicate precisely the same time. For this purpose Mr. Bain took for a standard time-keeper a clock of the best possible construction, placed in circumstances favourable to maintaining accuracy. The minute-hand of his clock, the instant that it pointed to the hour, made connection with a voltaic battery that brought into action a series of electro-magnets attached to the clocks to be regulated; one of them being fixed on the top of each clock. Its momentary action was made to collapse a pair of clippers, which in closing seized the minute-hand of the clock to which it was attached, and brought it to the hour point. Thus all the clocks in the series could be regulated every hour, for the collapse of the clippers pushed the hand forward if it were too late, or thrust it back if it had gained. Mr. Bain contemplated the application of this contrivance to all the public clocks of a town, by having wires laid down in the streets to connect them in one voltaic circuit. Such a plan would, however, have involved greater expense and trouble in its accomplishment than the object seemed to merit; but the regulation of any number of clocks in a large establishment might have been practicable by that means. We are not aware, however, that this mode of regulating clocks by electricity was ever adopted, and it has since been superseded by an arrangement made by Mr. Shepherd, junior, to be presently noticed.
Improving on this first application of electro-magnetism to the regulation of clocks, Mr. Bain afterwards employed the power to keep the clocks in action, so that each clock might be propelled by magnets alone, without any weight, and without the ordinary train of wheels.
Every one acquainted with the mechanism of a clock is aware that the weight communicates motion to a train of wheels, and that the movement is regulated by the vibration of a pendulum, which is acted on by the last wheel of the train. That wheel, called the escapement, is so formed, that each tooth catches in succession into a detent fixed on the pendulum near the point of suspension, which allows one tooth to pass at each double vibration. The pendulum, therefore, governs the movement of the train of wheels by checking the escapement, and allowing the teeth to pass one by one; and as pendulums of given lengths vibrate in given times, if their actions be not interfered with, the clocks will keep regular time. But the pressure of the escape-wheel against the detent, and the consequent friction, prevent the pendulum from acting freely. In the best made clocks there are special contrivances to detach the pendulum as much as possible from the wheels, and likewise to compensate for variations in the length of the pendulum by change of temperature.
In the clocks actuated by electro-magnetism, the movement of the pendulum is not maintained by repeated impulses of the escape-wheel, as in ordinary clocks, but by magnetic attraction; an electro-magnet being so arranged as to attract the bob of the pendulum in both directions alternately. In Mr. Bain's arrangement, the bob of the pendulum is formed of a hollow coil of covered copper wire, which, on the transmission of an electric current, becomes magnetic, and it is then attracted by several permanent magnets fixed in a hollow horizontal bar, over which the coil of wire moves. The accompanying diagram will serve to explain more clearly the parts of the clock on which the movement of the pendulum depends.
The pendulum rod, _B_, is made of wood, and the bob, _A_, consists of a hollow coil of thick copper wire covered with cotton, through which the hollow bar, _C C_, passes. Inside that bar there are several permanent magnets, packed on each side of the ends of the coil of wire, the poles of those on one side being the opposite of those on the other. In the diagram only one magnet on each side is represented, _n_ and _s_, to prevent confusion. The ends of the coil of wire are attached to the pendulum rod, and they are conducted up it so as to form connection with the wires of the voltaic battery, which are connected with gold studs inserted into a horizontal stage fixed to the clock-case. A small movable bridge, formed of wire, and having the ends tipped with gold or platinum, rests upon the stage, and is shifted from side to side by the pendulum. In these movements the gold points touch and slide over the gold studs in the stage, and thereby make and break contact with the voltaic battery, and alternately send and interrupt an electric current through the coil of wire.
Suppose, for instance, that the pendulum is about to rise to the right towards _s_, at which time the voltaic circuit is completed. The coil is, therefore, magnetic, and is attracted by the permanent magnet in _C_. As the pendulum approaches the end of its swing, it pushes the movable bridge away from the gold studs on which it rests, and thus breaks connection with the voltaic battery, and the pendulum descends unrestrained by the attractive force of the magnets. As the pendulum descends towards its lowest point, it shifts the bridge on to the metal studs on the other side, which are so disposed as to send a current through the coil in a direction opposite to the former, so that the poles of the voltaic battery are reversed, and the attractive force is exerted in drawing the pendulum towards the left hand. In this manner the power imparted to the coil, as the pendulum vibrates to and fro, produces a continuous repetition of the attraction on each side alternately, and maintains a constant action.
The only wheels required in a clock of this kind are those which turn the hands; and the motion is communicated from the pendulum to the seconds wheel by means of a small attached lever, working on a ratchet wheel. The minute and the hour hands derive their movements from the seconds wheel in the usual manner.
The voltaic battery employed to work Mr. Bain's clocks consists of a pair of large copper and zinc plates buried in the moist earth, which excite a sufficient amount of electricity to maintain the motion of the pendulum. A battery of this kind will remain in action a long time, and will serve to keep a clock going for several months. It is, indeed, a near approach to the attainment of perpetual motion, since nothing but the wearing away of the materials, or the accumulation of dust on the connecting points, seems to prevent the realization of that mechanical chimera.
There is a disadvantage attending the arrangement of Mr. Bain's clocks, arising from the attachment of the pendulum to the wheels; and as the moving force is derived directly from voltaic electricity, any variation in the power of the battery causes variation in the lengths of the vibrations, and produces irregularity. For the purpose of remedying these defects, Mr. Shepherd, junior, has adopted an arrangement which detaches the pendulum from the clock movement, and makes its vibrations altogether independent of the varying force of voltaic batteries.
In Mr. Shepherd's arrangement, the impulse of the pendulum is given by successive blows from a spring, which is drawn back and then liberated at each vibration. The hands of the clock are also moved by electro-magnets, by which means the impelling forces and the resistances encountered by the pendulum are always constant. By making the pendulum thus independent of the works, and employing it merely to make and break contract at regular intervals, any number of clocks in the same establishment may be set in motion, and kept exactly together, by a single pendulum.
The large clock over the principal entrance to the Great Exhibition was on this construction. It would have been impossible, with any approach to regularity, to have moved hands of that size, exposed as they were to the wind, unless the pendulum had been independent of such resistances.
Electro-Magnetic Clocks have not yet come into general use, partly owing to imperfections in the battery connections, which occasionally put a stop to their movements, but principally on account of the high prices charged by the patentees. As no trains of wheels are requisite in an Electro-Magnetic Clock, it might be manufactured very cheaply; and when the price is reduced to its proper standard, and the trifling practical defects are remedied, these clocks may possibly supersede others.
ELECTRO-METALLURGY.
The electrotype, electro-gilding and plating, and the other applications of the deposition of metals from their solutions, by the agency of voltaic electricity, had their origin in the chance observation of peculiarities in frequently repeated experiments. In this, as in most other inventions, there are contending claimants for priority; but there is little merit due to any of the first discoverers of the process, who seem to have been guided altogether by accident. It seems strange, now, on observing the extensive use that is made of the deposition of metals, that it should have remained so long unapplied after the principle had been known.
The "revivification," as it was called, of metals from their solutions by voltaic electricity, was known at the beginning of the present century; for, in 1805, Brugnatelli, an Italian chemist, gilded a silver medal by connecting it with the negative pole of a voltaic battery, when immersed in a solution of ammoniuret of gold. It did not occur to him, however, that any use could be made of that mode of gilding, and the experiment had no result.
Nothing further was done, even experimentally, towards advancing the art of electrotyping, until Mr. Spencer, of Liverpool, when experimenting with a Daniell's battery, in 1837, accidentally coated a penny piece with copper; and when the thin sheet of metal was removed, he found on it an exact counterpart of the head and letters of the coin. Even this did not suggest any useful application; nor was it until, by a further accident, a drop of varnish fell on the copper of the negative pole, and showed that no deposition took place on the part so covered, that the idea occurred to him of turning the deposition of the copper to account. The method he adopted of doing so was to cover a copper plate with varnish or wax, and to etch a design through the covering. By then exposing the plate to the action of a solution of sulphate of copper, when in connection with the negative pole of a voltaic battery, the metal was deposited in the lines drawn through the varnish, and a design in relief was fixed to the copper. This slight advance in the art was not made known until it was announced, in 1839, that Professor Jacobi, of St. Petersburg, had made application of the same process. Mr. Spencer, indeed, was forestalled, even in this country, by Mr. Jordan, a printer, who published an account in the _Mechanics' Magazine_ for May, 1839, of a method of making copper casts by the deposition of copper from its solution. In the autumn of the same year, however, Mr. Spencer exhibited to the British Association several more perfect specimens of electrotyping, that showed the process might be rendered valuable; and from that time rapid progress was made in bringing it into practical operation in a variety of ways.
The deposition of copper from its solution, when under the action of voltaic electricity, is not produced by the decomposition of the sulphate of copper, as might be supposed, but by the decomposition of the water that acts as the solvent of the metallic salt. Thus, when two platinum wires from the poles of a voltaic battery are introduced into acidulated water, hydrogen gas is disengaged at the wire connected with the negative pole, and oxygen at the other; but when a solution of sulphate of copper is substituted for water, the hydrogen that is disengaged combines with the oxygen that held the copper in solution, and the metal is liberated. The copper thus liberated from its combination with the oxygen is deposited, in a pure metallic state, on the surface connected with the negative pole of the battery.
The simplest illustration of electro-metallic deposition is obtained by immersing a silver spoon and a strip of zinc into a solution of sulphate of copper. So long as the two metals are kept apart, no change takes place on the silver, but on bringing them into contact, voltaic action immediately commences, and a coating of copper is deposited upon the spoon, and adheres firmly to the metal. If the action be continued, and the supply of copper be maintained by the addition of fresh crystals of the sulphate, the coat of copper may be increased in thickness to almost any extent.
The first applications of the discovery were directed to the copying of medals and coins. An impression of the metal was obtained in fusible metal, which is an alloy composed of tin, lead, and bismuth, melted together in the proportions of two of the latter to one each of the former. This alloy expands on cooling, and thus affords a very sharp impression of the medals; and as it melts at a low temperature, it may be easily removed after the copper coating has been deposited upon it.
An electrotype mould, obtained directly from the medal, is, however, more sharp in its definition than an impression, and is therefore preferable, when circumstances admit of its being so taken. For that purpose, the surface whereon the deposition is to be made is smeared over with sweet oil, or with black lead. It is then carefully wiped with cotton wool, but a minute quantity of the oil will still remain, sufficient to prevent the metal from adhering.
A simple form of apparatus for the electrotype process is shown in the accompanying diagram.
An earthenware jar, _a_, serves to hold the solution of copper, which should be maintained in a saturated state by the addition of crystals of the salt. A porous tube, _b_, holds a rod of amalgamated zinc, to the top of which a binding-screw is soldered. The copper mould or medal, _c_, is suspended in the solution by a wire, which is held tight by the binding-screw, _d_. The porous jar is then filled to the same height as the copper solution in the jar with diluted sulphuric acid, in the proportions of one of acid to twenty of water. Voltaic action immediately commences, and the copper will continue to be deposited from the solution as long as the supply of fresh crystals of sulphate of copper is continued. In about twenty-four hours the coating of copper will be as thick as a thin card, and it may be then removed. When detached from the medal, it will be found to be an exact counterpart, in the minutest details, of the original; those parts of the medal which are in relief being, of course, the reverse in the mould.
The extreme minuteness and delicacy of the electrotype process is strikingly exemplified in its application to the transference of engraved copper-plates. A highly finished engraved copper-plate has a film of metal deposited over its whole surface, which, when detached, exhibits all the lines that are cut into the copper-plate in relief. That electrotype cast then serves as the mould for further depositions, in which every line in the original engraving is so perfectly developed, that it is impossible to detect a difference in the impressions taken from the two plates. By this means any number of casts may be made and worked from, whilst the original is preserved uninjured. The objection to this application is that the metal deposited is not so hard as the hammered plates, and will not, therefore, bear the wear and tear of copper-plate printing so well as the plates made by hand.
It was at one time supposed that the depositing of metal on surfaces, by voltaic action, might be applied to the manufacture of numerous kinds of copper articles without manual labour. For this purpose, casts were made of plaster of Paris, which were covered with black lead, to give them the property of conducting electricity, and the metal was then deposited upon them. But, independently of the practical difficulties attending the operation, it was found that the metal was not sufficiently hard, and the cost of the requisite voltaic batteries rendered the economy of the process questionable.