In the following year Messrs. Cooke and Wheatstone so far simplified the arrangements of their needle telegraph as to make all the requisite signals with two needles. With a single combined battery and two wires six primary signals are thus obtained; and by repeating the deflections and combining the action of the two needles, all the letters can be readily and quickly indicated. A single needle instrument was invented by Messrs. Cooke and Wheatstone, but as there are only two primary signals, one to the right and one to the left, the deflections are necessarily repeated more frequently, and the transmission is consequently more slow. The accompanying diagram represents the alphabet of the single needle instrument. The deflections for each letter commence in the direction of the short marks, and end with the long ones. Thus, to indicate the letterR, the needle is first deflected once to the left and then once to the right; and the letterDhas the deflections reversed, beginning with one to the right and ending with one to the left. In no instance does it require more than fourdeflections to indicate a single letter, yet the transmission with the double needle is found so much quicker that the single needle instrument is only rarely used.
At the end of each word, it is customary for the clerk at the receiving station to indicate, by a deflection of the needle to the right, that he understands, or by a deflection to the left, that he does not understand, and in the latter case the word is repeated. In the early days of the Electric Telegraph, the transmission of 40 letters a minute with the double needle instrument was considered quick work; but the practised clerks will now transmit one hundred letters in that time, which is as fast as any person can write with pen and ink.
Since the invention of the double and single needle telegraphs there have been many modifications in the instruments, to make them work more promptly and with less vibration; but in all essential parts the telegraphs of Messrs. Cooke and Wheatstone remain unaltered, and continue to be generally used in this country.
Of the numerous other telegraph instruments that have been invented since 1837, that of Mr. Morse is in most general use, especially on the Continent and in America. Mr. Morse, indeed, claims to be the first inventor of a practical Electric Telegraph; for, according to his statement, he, in 1832, invented a telegraph, which was in principle the same as the one now in use. It was not, however, till September, 1838, that he made his instrument known in Europe,by sending a description of it with a model to the Academy of Sciences at Paris. Mr. Jackson, an American, disputed with Mr. Morse for the honour of the invention, and when the latter asserted that he had described his telegraph in 1832, to some passengers on board a packet-boat, Mr. Jackson affirmed that it was he who described it on that occasion, and that Mr. Morse, being present, got the idea from him. It is painful and difficult to decide when we find two claimants thus directly in opposition to each other, and mutually preferring charges of falsehood and fraud. The only safe guide in such cases is to refer to the earliest published and authentic descriptions of the inventions; and, following that guidance, the invention of what is called Morse's telegraph must be attributed to him whose name it bears; but we must, according to the same rule, date it several years later than 1832.
Mr. Morse's telegraph is a recording instrument, that embosses the symbols upon paper, with a point pressed down upon it by an electro-magnet. The symbols that form the alphabet consist of combinations of short and long strokes, which by their repetitions and variations, are made to stand for different letters. Thus a stroke followed by a dot signifies the letterA; a stroke preceded by a dot, the letterB; a single dot, the letterE; and in this manner the whole alphabet is indicated, the number of repetitions in no case exceeding four for each letter. The letters and words are distinguished from one another by a longer space being left between them than betweeneach mark that forms only a part of a letter or of a word. The annexed diagram represents the symbols for the whole alphabet.
The mechanism of this telegraph instrument is very simple. The transmitter is merely a spring key, like that of a musical instrument, which, on being pressed down, makes contact with the voltaic battery, and sends an electric current to the receiving station. The operator at the transmitting station, by thus making contact, brings into action an electro-magnet at the station he communicates with, and that pulls down a point fixed to the soft iron lever upon a strip of paper that is kept moving by clockwork slowly under it. The duration of the pressure on the key, whether instantaneous or prolonged for a moment, occasions the difference in the lengths of the lines indented on the paper. A single circuit is sufficient for this telegraph, and a boy who is practised in the use of the instrument will transmit nearly as many words in a minute as can be sent by the double needle telegraph with two wires.
The working of Mr. Morse's telegraph, it will be observed, depends altogether upon bringing into action at the receiving station an electro-magnet of sufficient force to mechanically indent paper. Nowthe resistance to the passage of electricity along the wires diminishes the quantity transmitted so greatly, that at long distances it would be almost impossible to obtain sufficient power for the purpose, if it acted directly. To overcome that difficulty, an auxiliary electro-magnet is employed. The electro-magnet which is directly in connection with the telegraph wire is a small one, surrounded by about 500 yards of very fine wire, for the purpose of multiplying as much as possible the effect of the feeble current that is transmitted. The soft iron keeper, which is attracted by that magnet, is also very light, so that it may be the more readily attracted. This highly sensitive instrument serves to make and break contact with a local battery, which brings into action a large electro-magnet, and as the local battery and the magnet are close to the place where the work is to be done, any required force may be easily obtained. By this means the marks may be impressed on the paper at distances of 400 miles or more apart.
This is a very efficient and remarkably simple telegraph, and as it operates with a single wire, it has completely supplanted the needle telegraph on the Continent; though the liability to error, common to all manipulated telegraphs, is considerably increased by this mode of transmission, nor can unintelligible signals be indicated and corrected so readily as by the needle instrument.
There have been several modifications of Mr. Morse's telegraph, for the purpose of increasing the rapidity of its action and the distinctness of the marks.The most important of these was made by Mr. Bain, who in 1847 applied for this purpose the method of impressing the symbols on paper by electro-chemical decomposition. Mr. Davy had, in 1843, taken out a patent for the application of electro-chemical marks to telegraphic purposes, but his method was not sufficiently practical to be brought into use. Mr. Bain adopted an alphabet of short and long strokes, similar to that of Mr. Morse; but instead of making and breaking contact by a key pressed down by the finger, he punched holes in a strip of paper, corresponding in lengths and positions to the marks intended to be transmitted. A small metal spring, connected with the voltaic battery, pressed upon a metal cylinder attached to the telegraph wire, and when the spring and cylinder touched, an electric current was transmitted. The strip of punched paper was placed upon the cylinder so as to interrupt the circuit, excepting in the parts where the apertures allowed the spring to make contact; therefore when the strip of paper was moved along, an electric current was transmitted through the apertures, and it was stopped when the paper intervened. At the receiving station, paper well moistened with a solution of prussiate of potass and nitric acid was placed upon a corresponding cylinder to receive the message, and a piece of steel wire was kept steadily pressed upon it as it moved along. The action of the electric current at the parts where it was transmitted caused the acid to enter into combination with the steel, and the consequent deposition of iron on the paper was instantlyconverted by the prussiate of potass into Prussian blue. On the parts where the electric current was interrupted no action took place, and thus numbers of short and long marks were made on the paper, corresponding to the lengths of the apertures on the prepared message. A representation of the punched paper for transmitting the word "Bain" is shown in this diagram.
As electro-chemical action takes effect much more rapidly than the mechanical movement of an indenting point, Mr. Bain's telegraph could work much faster than Mr. Morse's. We have been informed that as many as 1,000 letters per minute have occasionally been transmitted by this means from Manchester to London. The disadvantage attending that mode of transmission arises from the tedious process of punching the message preparatory to transmission; and though circumstances may arise in which it would be of great importance to adopt this rapid system of transmission with a single wire, it has been yet but little used in this country by the Electric Telegraph Company, who purchased Mr. Bain's patent for £10,000.
Another modification of Mr. Morse's telegraph, which has been more extensively adopted in England, consists in merely substituting marks made on paper by electro-chemical decomposition for those indented by pressure. It has been found desirable in practice,however, to introduce an auxiliary electro-magnet, called a "picker," for making and breaking contact, by which arrangement the dotted marks can be made by a local battery, and any required amount of electric power be obtained. The marks produced in this manner are more distinct, and are more quickly made, than by mechanical pressure. By a more recent application of Mr. Morse's system, the marks are made on paper with ink flowing through a glass pen, in the same manner as in the telegraph of M. Schweigger, already noticed. As the strip of paper is moved along, a continuous line is thus drawn on the paper. When no signals are being transmitted the line is straight, but when an electric current is sent through the wire, it brings into action an electro-magnet, which attracts the penholder on one side, and alters the direction of the mark. The transmission is effected by making and breaking contact with a key, and the continuance of the divergence of the mark from its normal direction is regulated by the duration of pressure on the key. The symbols are thus made by deviations from the straight line, of different lengths and of varied combinations. Practical application alone can determine whether this mode of making the marks possesses any advantage over Mr. Morse's original plan. The patent for this telegraph was granted to Mr. Wilkins in 1854, but a similar instrument, applied to the notation of astronomical observations, was shown in the American department of the Great Exhibition of 1851.
The recording telegraph instruments hitherto noticedimpress on the paper only hieroglyphical symbols, which require long practice to decipher readily. It has, from the first practical application of the invention, been considered highly desirable that the letters of the alphabet should be indicated and printed in their proper forms, so that the momentary transmission of an electric current should leave behind a durable impression that could be read without difficulty. Professor Wheatstone and Mr. Bain separately attempted to accomplish this desired object by the invention of Printing Telegraphs, which print messages from types. It is a question in dispute which of them was the first to design a telegraph of this kind. In 1845, Mr. Bain had a printing telegraph in operation experimentally on the South-Western Railway, for a distance of seven miles, and we are not aware that Professor Wheatstone ever succeeded in working his printing instruments when separated at a distance from each other. In principle, both inventions were similar. A wheel, into the periphery of which were inserted types of the twenty-six letters, was made to rotate in close proximity to a piece of paper, over which was placed a blackened surface that would leave a mark on the paper when pressed upon. When the required letter came opposite the paper, the type-wheel was stopped and forced against it, so that the letter was impressed, and the black from the interposed surface marked the form of the type. The paper was then moved forward to leave space for the next letter, and thus a continuous message could be printed. The objection to these instruments was theuncertainty of stopping the type-wheel at the proper point, so as to avoid printing wrong letters; and when the instruments became thus irregular, they continued so till they were again adjusted. This difficulty has since been overcome; and by the combined efforts of Mr. House in America, and of Messrs. Brett in this country, the printing telegraph has attained a high degree of perfection. The mechanical arrangements of the instrument, though very complex, consist essentially, like those of Mr. Bain and Professor Wheatstone, in having a type-wheel, which, by the action of the operator at the transmitting instrument in making and breaking contact, moves or stops at the required point, and the letters are printed by forcing the paper against the type by an electro-magnet. The movements of the type-wheel are regulated by an electro-magnet, and one great improvement introduced by Mr. Brett prevents the continuance of error, should any be made during transmission, by bringing the type-wheel to its first position after printing each letter, so that if a wrong letter be printed, the subsequent letters will not continue erroneous. This printing telegraph works with a single wire, but its operation is rather slow.
The last recording telegraph we shall notice is the one invented by the author, which transmits copies of the handwriting of correspondents. The communication to be transmitted is written upon tin foil, thinly coated with varnish, with a pen dipped in an ink composed of caustic soda and colouring matter. The alkali detaches the varnish, and when the surface iswashed over with a wet sponge, the metal is exposed on those parts written upon, the writing appearing metallic on a dark ground. The message is then placed round a metal cylinder that is connected with the line wire from the receiving station. A brass point, in connection with the voltaic battery, lightly presses on the message as the cylinder rotates, so that the electric circuit is made and broken through the message as it passes under the connecting point, the coating of varnish on the foil being sufficient to interrupt the electric current in those parts where the point is resting upon it. On a corresponding cylinder in the electric circuit, at the receiving station, paper moistened with a solution of prussiate of potass and nitrate of soda is placed to receive the message; and it is pressed upon by the point of a steel wire, in connection with the communicating wire. The accompanying diagram will assist in explaining the arrangement.
The cylinder of the instrument is shown ata;bis the metal style connected by the wiregwith one of the poles of the voltaic battery;ois the arm which holds the style and serves to insulate it from the rest of the apparatus;cis a fine screw on which that arm traverses as the cylinder revolves;d dare cog-wheels to turn the screw. The speed of the instrument is regulated by the fane;fis the impelling weight, andhthe wire connected with the distant instrument. The receiving and the transmitting instruments are alike, the only difference between them being that the style of the copying instrument is steel instead of brass wire.
As the cylinderais connected by the wirehwith the distant instrument, and through it with one of the poles of the voltaic battery, the electric circuit is completed by passing fromgthrough the tin foil message, or through the paper placed on the cylinder. This will be the case whenever the style of the transmitting instrument is pressing on the metallic writing; and at those times the electro-chemical action of the voltaic current will produce a blue mark on the paper of the receiving instrument, by the deposition of iron and its combination with the prussiate of potass. The circuit will in like manner be interrupted wheneverthe pointbpresses on those parts of the message where the varnish is not removed; and thus, as the two cylinders revolve, there will be a succession of small blue marks on the parts where the writing allows the electric current to pass. As the arms that carry the points traverse on screws, they are drawn along as the cylinders rotate, so as to press on fresh parts of the message and of the paper at each revolution. The steel point would therefore draw a series of spiral lines on the paper, if the electric current were not interrupted; but the interposition of the varnish breaks those lines, and as the point passes over different portions of the letters at each revolution of the cylinder, the marks and the interruptions on the paper correspond exactly with the forms of the letters, and thus produce a copy of the writing placed upon the receiving cylinder, in blue characters on a yellowish ground. Or the message may be written on unprepared tin foil with a pen dipped in varnish; in which case the writing will be copied in white characters on a ground of dark lines, as in the accompanying specimen,Abeing the writing on tin foil, andBthe message received.
It is essential to the perfect working of the copying telegraph that the corresponding instruments should rotate exactly together. This is effected by an electromagnetic regulator, which being put in action by one instrument, governs the movements of the distant instrument with the greatest exactness, as proved at a distance of 300 miles.
It might be supposed, as the points must traverseseveral times over the same line of writing to copy it, that the process is a slow one; but in consequence of the rapidity with which the cylinders revolve, this is not the case. The ordinary speed is one rotation in two seconds, and at that rate three lines of writing, containing sixty words, would be copied in one minute, which is three times as fast as an expeditious penman can write.
The advantages proposed to be gained by the copying telegraph, in addition to its increased rapidity of transmission, are the authentication of telegraphic correspondence by the signatures of the writers, freedom from the errors of transmission, and the maintenance of secrecy. As a special means of obtaining secrecy, the messages may be received on paper moistened with a solution of nitrate of soda alone, in which case they would be invisible until brushed over with a solution of prussiate of potass, to be applied by the person to whom the communication is addressed.
Professor Wheatstone has recently contrived an improvement in his index telegraph, which was described by Professor Faraday in a lecture at theRoyal Institution in June last. Its chief merit, however, consists in the beauty of the mechanism, for it is essentially the same as the index telegraphs he and others have previously invented, with the substitution of magneto-electricity for the moving force.
Having now traced the history of the invention of the instruments by means of which messages may be transmitted, it becomes necessary to describe the methods employed for making the electrical connection from one place to another. This part of the electric telegraph system is, after all, the most essential to its efficient working, and bears the same relation to the transmitting instruments that the structure of a railroad does to locomotive engines in the system of railway conveyance.
The fact that an electric current might be sent through a long circuit had been established by Dr. Watson, in conjunction with other Fellows of the Royal Society, in 1747, when they sent the charge of a Leyden jar through two miles of wire, supported upon short sticks driven into the ground; the wire at each terminus being connected with the earth for the return current. This method of insulation and conduction fully answered the purpose, and served to determine the great velocity with which electricity is transmitted, for no perceptible interval occurred between the discharge of the Leyden jar at one end of the circuit, and its effect at the other extremity.
Mr. Ronalds made the next experiment on an extensive scale, by insulating eight miles of wire in glass tubes, the wire being carried backwards and forwardsfor that distance on his lawn at Hammersmith. That mode of insulation was found very efficient. It was, indeed, too perfect, for the difficulty arose of discharging the electricity from the wire after the charge had passed through it.
The length of telegraphic communication established at Munich, in 1837, by Dr. Steinheil, was an important practical advance in the system of extending and insulating the wires, and deserves consideration, not only from the extent to which it was carried into practical operation, but from the circumstance that the earth was employed to form the return circuit. The wires appear to have been carried through the city by extending them from the church towers and other elevated buildings. That plan, indeed, presents so many facilities for passing telegraph wires through towns, that it is not improbable it may be ultimately adopted in this country.
Though the conducting power of the earth was thus early made use of for one-half of the circuit, the fact seems to have been unknown in England at the time of laying down the telegraph wires to Slough in 1845, for a separate wire was then used for the return current. Some years afterwards, indeed, Mr. Bain laid claim to the discovery; but the fact that the conducting power of the earth had been previously applied to the purpose by Dr. Steinheil has been incontestably proved.
In the early stages of the practical application of electric telegraphs in this country, Mr. Cook took an active part in overcoming the numerous difficultiesattending the proper protection and insulation of the wires. In the first instance, the plan of burying the wires in trenches was tried, but with very indifferent success, as the asphaltum and other resinous substances with which it was attempted to insulate them were inadequate for the purpose, and allowed the electricity to escape from wire to wire. The method of supporting the wires on tall posts was then adopted by Mr. Cooke, the wires being insulated from the posts at the points of suspension, by passing them through quills. Various improvements have since been made in the insulators, and the plan most in favour at present is to pass the wires through globular earthenware or glass insulators, attached to the posts, as shown in the annexed diagram. The wires themselves are about one-sixth of an inch in diameter; they are made of iron coated with zinc, or galvanized, as it is termed, to protect them from rust.
Notwithstanding the great care taken to insulate the wires at the posts, a large quantity of the electricity escapes in wet weather, and returns to the battery without having reached the most distant stations, and thus not unfrequently the communications are interrupted. The author is of opinion that the loss ofelectricity in wet weather is occasioned rather by communication from one wire to another through the moist atmosphere, than by defective insulation at the posts. In confirmation of this opinion it may be stated, that he has experimentally determined that a working electric current might be transmitted from London to Liverpool, if all the points of attachment were connected by water with the surface of the ground, provided that the rest of the wire wereinsulated.7
The use of gutta percha as an insulating covering for wire has given rise to a new era in telegraphic communication. Gutta percha is an excellent insulator, and wire covered with two coatings of that material, about one-sixteenth of an inch each, is so far protected, that 100 miles of it immersed in water transmits an electric current from a powerful voltaic battery with very trifling loss. This perfection in insulation has greatly facilitated the establishment of telegraphic communication between England and the Continent. The first attempt to establish a submarine circuit between Dover and Calais took place on the 28th of August, 1850. A single copper wire, about the thickness of a common bell wire, coated thickly with gutta percha, was laid across the English Channel experimentally, without any protection. It proved sufficient for the transmission of an electric current, and several messages were sent through it between Dover and Calais; but it was far too feeble to resistthe action of the waves, and the following day it was cut through by friction against the rocks, and the communication was stopped.
The plan afterwards adopted for a permanent submarine line was to enclose five similar wires in a hollow iron wire cable. The wires were first slightly twisted, to prevent them from being broken when stretched. They were then covered with hempen yarn, to protect the gutta percha from attrition, and they were thus introduced into the hollow cable, of which they formed the core. The accompanying woodcut represents this structure of the cable; the five twisted wires are shown atC;Brepresents the same covered with hemp yarn; andAa portion of the completed cable, constructed of thick iron wire galvanized. This cable has now been laid down for seven years, and with perfect success. Its strength has often been severely tested, as it has been sometime drawn up by ships' anchors, and considerably strained; but it has not been broken, and the insulation is almost perfect. The success of this submarine cable has induced the extension of that means of communicating with the Continent, and similar submarinetelegraph cables have been laid down from Dover to Ostend, from Harwich to the Hague, from Scotland to Ireland, and across the Mediterranean Sea as far as Malta. The weight and the cost of those cables present a serious obstacle to their adoption in forming a telegraphic communication with America; and when it was determined to attempt to establish electrical connection with the New World, a different form of cable was adopted. The conductor of the electric current in the Atlantic cable is composed of seven strands of fine copper wire twisted together, the aggregate thickness of which is not greater than the single copper wire of other submarine cables. This fine copper cord is covered carefully with gutta percha; it is then coated with tarred hemp, and is protected externally by an iron wire rope, composed of numerous strands of fine wire. The form and exact size of the cable are shown in the accompanying drawing and section. The central dots in the section are the conducting wires round which are the gutta percha and hemp, and the outer rim represents the iron wire casing.
The successful laying down of so frail a cable, after many failures, affords good ground for hoping that,with the experience already gained, subsequent efforts will prove more satisfactory and much less expensive than this first attempt to establish telegraphic communication with America. The most questionable part of the problem has, indeed, been already solved; for the transmission of electric signals, through that length of submerged wire, was at one time doubted; and though the communication through the present cable has ceased, it has sufficiently established the fact, that telegraphic communication with America is a practicable undertaking.
The excellent insulation obtained by means of gutta percha covered wires has caused a return to the original plan of burying the wires in trenches in the ground. The British and Submarine Telegraph Company make all their communications by that means; the number of coated wires required being enclosed in iron tubes, and laid in the ground along the common roads. That plan is, however, attended with considerable disadvantages. In the first place, the cost of the coated copper wire is more than quadruple that of galvanized iron wire; and though copper, compared with iron, offers only one-seventh part the resistance to the transmission of electricity, yet the thin wire employed is scarcely equal in conducting power to the galvanized iron wire usually supported on posts. The quantity of electricity transmitted is therefore less, and the comparative intensity of it is greater.
Another difficulty attending the use of insulated wires buried in the ground arises from a very peculiar condition of electrical conduction, that could scarcelyhave been anticipated. The wire, coated with gutta percha, and surrounded externally with water or with moist earth, becomes an elongated Leyden jar; the gutta percha representing the glass, the wire the inside coating, and the water the conducting surface outside. Thus, when electricity is transmitted through such a medium, a portion of the charge is retained after connection with the battery has been broken. This effect increases with the length of the wire and the intensity of the current; and it materially interferes with the working of many telegraph instruments. In some experiments with the copying telegraph at the Gutta Percha Works in the City Road, it was found that through a circuit of 50 miles of wire immersed in water, the mark made by electro-chemical decomposition on paper had a tendency to become continuous; so that instead of ceasing to mark, when the varnish interrupted the current, a line was drawn continuously on the paper, though the stronger marks where the current passed were sufficient to make the writing legible. The retention of the charge was also shown still more remarkably by the explosion of gunpowder by the electricity retained in the wire half a minute after connection with the battery had been broken. It is owing to the retention of the electricity by the wire that the slowness with which the messages through the Atlantic cable were transmitted is to be attributed, and not to the length of the cable. The rate of one word a minute was the average speed of transmission when the first messages were sent through the wire. The effect of theretardationof theelectric current is comparatively insignificant and were it not for the peculiar action of the surrounding water, the messages might have been transmitted twelve times faster than they were.
The cost of constructing a telegraphic line has greatly diminished with the increased facilities of insulating the wires, and since the expiration of patents, which conferred a monopoly on certain plans of doing so. The cost to the Great Western Railway Company for a line of six wires to Slough, was £150 per mile, with comparatively low and slender posts and very imperfect insulation. The cost of the same number of wires at the present day would not be one-half that sum, with thicker wires and better insulation.
It is customary in England to restrict the suspension of telegraphic wires to railways, from the notion that the protection of railways is necessary to prevent wilful damage to the wires; and as the Electric Telegraph Company have made exclusive arrangements with all the railway companies out of London, the competing telegraph companies have preferred to lay their wires underground rather than incur the supposed risk of damage to the wires if suspended from posts on common roads, though by this means the cost of construction is at least quadrupled. The protection which railways afford is, however, more imaginary than real, for any one inclined to interrupt the communication could easily do so; and if on common roads proper precautions were taken in fixing the posts, and a heavy penalty were imposed on wilful offenders, the common roads and open fields would, there canbe little doubt, offer as safe a course for the telegraphic wires as railways.
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 moreclearly understood by reference to the accompanying diagram.
The wire fromC, which is the copper pole of the voltaic battery, is connected with the instrumentA; the electric current is then transmitted along the wireDto the receiving instrumentB; thence it is transferred to the earth-plateE, passes through the earth to the corresponding plateE´, which is connected withZ, the zinc pole of the battery. When a communication is returned fromBtoA, a similar arrangement is made; the wires connected with the instruments being so arranged as to bring into action a voltaic battery atB, and to throw out of circuit the one atA; 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 cannotmingle, 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 lettersABrepresent the wires making communications between the batteriesDandE, and the telegraph instrumentsIOat the receiving station. The electricity from the copper end of the batteryDwould be conducted alongAthrough the instrumentI, and by the wireKto the earth-plateH. It would be then transmitted through the earth on its return to the battery, in the direction of the arrows, to the other earth-plateG, and thence it would find its way to the zinc pole of the batteryD, and complete the circuit. In the same manner, the electric current from the copper end of the batteryEwould be transmitted through the wireB, and would complete its current also by means of the earth-platesGH, and would traverse the course indicated by the arrows, and return to the zinc end ofE. Though both electric currents traverse the same wire from the instrumentsIOto the earth-plateH, and are thence transmitted through the earth to a single plate,G, at the transmittingstation, 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 tointermix.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 communicationis 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.
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; oneof 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, calledthe 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,nands, 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.