William Thomson, Lord Kelvin (orn: June 26, 1824 / died: Dec 17, 1907)
|Sir William Thomson, 1st Baron Kelvin of Largs was
a mathematical physicist who did important work in thermodynamics. In recognition
of his achievements, he was made the first Baron Kelvin of Largs and was
commonly known as Lord Kelvin. He was buried in Westminster Abbey, London.
Thomson was born in Belfast, Ireland. His father, Dr. James Thomson, son of a Scots-Irish farmer, had educated himself at Glasgow University while working as a teacher. Appointed head of a school in connection with the Royal Academical Institute, he later obtained the professorship of mathematics in the Institute.
Thomson became a man of public note in connection with the laying of the first Atlantic cable. After William Cooke and Charles Wheatstone had introduced their working telegraph in 1839, the idea of a submarine line across the Atlantic Ocean began to be thought of as a possible triumph of the future.
|Samuel Morse proclaimed
his faith in it as early as the year 1840, and in 1842 he submerged a wire,
insulated with tarred hemp and india rubber, in the water of New York harbour,
and telegraphed through it.
The following autumn Wheatstone performed a similar experiment in Swansea Bay. A good insulator to cover the wire and prevent the electricity from leaking into the water was requisite for the success of a long submarine line. India rubber had been tried by Jacobi, the Russian electrician, as far back as 1811. Luckily another gum which could be melted by heat, and readily applied to the wire, made its appearance. Gutta-percha, the adhesive juice of the Isonnandra gutta tree, was introduced to Europe in 1842 by Dr. Montgomerie, a Scotch surveyor in the service of the British East India Company. Twenty years before he had seen whips made of it in Singapore, and believed that it would be useful in the fabrication of surgical apparatus. Faraday and Wheatstone soon discovered its merits as an insulator, and in 1845 the latter suggested that it should be employed to cover the wire which it was proposed to lay from Dover to Calais. It was tried on a wire laid across the Rhine between Deutz and Cologne. In 1849 Mr. C. V. Walker, electrician to the South Eastern Railway Company, submerged a wire coated with it, or, as it is technically called, a gutta-percha core, along the coast off Dover.
The following year, John Watkins Brett laid the first line across the English Channel. It was simply a copper wire coated with gutta-percha, without any other protection. The experiment served to keep alive the concession, and the next year, on November 13, 1851, a protected core or true cable was laid from a Government hulk, the Blazer, which was towed across the Channel. Next year Great Britain and Ireland were linked together. In May, 1853, England was joined to Holland by a cable across the North Sea, from Orfordness to the Hague. It was laid by the Monarch, a paddle steamer which had been fitted for the work.
The design of the first transatlantic cable was a subject of experiment by Professor Morse and others. It was known that the conductor should be of copper, possessing a high conductivity for the electric current, and that its insulating jacket of gutta-percha should offer a great resistance to the leakage of the current. Moreover, experience had shown that the protecting sheath or armour of the core should be light and flexible as well as strong, in order to resist external violence and allow it to be lifted for repair. There was another consideration, however, which at this time was rather a puzzle. As early as 1823, Francis Ronalds had observed that electric signals were retarded in passing through an insulated wire or core laid under ground, and the same effect was noticeable on cores immersed in water, and particularly on the lengthy cable between England and the Hague. Faraday showed that it was caused by induction between the electricity in the wire and the earth or water surrounding it. A core, in fact, is an attenuated Leyden jar; the wire of the core, its insulating jacket, and the soil or water around it stand respectively for the inner tinfoil, the glass, and the outer tinfoil of the jar. When the wire is charged from a battery, the electricity induces an opposite charge in the water as it travels along, and as the two charges attract each other, the exciting charge is restrained. The speed of a signal through the conductor of a submarine cable is thus diminished by a drag of its own making. The nature of the phenomenon was clear, but the laws which governed it were still a mystery. It became a serious question whether, on a long cable such as that required for the Atlantic, the signals might not be so sluggish that the work would hardly pay. Faraday told Field that a signal would take 'about a second,' and the American was satisfied; but Professor Thomson's the law of retardation cleared up the matter. He showed that the velocity of a signal through a given core was inversely proportional to the square of the length of the core. That is to say, in any particular cable the speed of a signal is diminished to one-fourth if the length is doubled, to one-ninth if it is trebled, to one-sixteenth if it is quadrupled, and so on. It was now possible to calculate the time taken by a signal in traversing the proposed Atlantic line to a minute fraction of a second, and to design the proper core for a cable of any given length.
The accuracy of Thomson's law was disputed in 1856 by Dr. Edward O. Wildman Whitehouse, the electrician of the Atlantic Telegraph Company, who had misinterpreted the results of his own experiments. Thomson disposed of his contention in a letter to the Athenaeum, and the directors of the company saw that he was a man to enlist in their adventure. The young Glasgow professor threw himself heart and soul into their work. He helped them out of all their difficulties. In 1857 he published in the Engineer the whole theory of the mechanical forces involved in the laying of a submarine cable, and showed that when the line is running out of the ship at a constant speed in a uniform depth of water, it sinks in a slant or straight incline from the point where it enters the water to that where it touches the bottom. To these gifts of theory, electrical and mechanical, Thomson added a practical boon in the shape of the reflecting galvanometer, or mirror instrument. This measurer of the current was infinitely more sensitive than any which preceded it, enabling the electrician to detect the slightest flaw in the core of a cable during its manufacture and submersion. Moreover, it proved the best apparatus for receiving the messages through a long cable. The Morse and other instruments, however suitable for land lines and short cables, were all but useless on the Atlantic line, owing to the retardation of the signals; but the mirror instrument sprang out of Thomson's study of this phenomenon, and was designed to match it. Hence this instrument, through being the fittest for the purpose, allowed the first Atlantic cables to be worked on a profitable basis.
There were several unsuccessful attempts before the cable was finally laid by the Great Eastern. On his return home, Thomson was among those who received the honour of knighthood for their services in connection with the enterprise. By his theory and apparatus he probably did more than any other man, with the exception of Field, to further the Atlantic telegraph. It was thanks to his inventions, the mirror instrument of 1857 and the siphon recorder of 1869, that messages through long cables are so cheap and fast, and, as a consequence, that ocean telegraphy became common.
The siphon recorder was a masterpiece of invention. As used in the recording or writing in permanent characters of the messages sent through long submarine cables, it became the acknowledged chief of 'receiving instruments' -- apparatus which interpret the electrical condition of the telegraph wire into intelligible signals. Like other mechanical creations, its growth in idea and translation into material fact was a step-by-step process of evolution, culminating at last in its great fitness and beauty. The development of telegraphy called into existence a variety of receiving. But all these instruments had one great drawback for delicate work, and were next to useless for long cables. They required a certain definite strength of current to work them. Most of the moving parts of the mechanism were comparatively heavy, and unless the current was of the proper strength to move them, the instrument was useless. On submarine cables, the current is slow and varying. It travels along the copper wire in the form of a wave or undulation, and is received feebly at first, then gradually rising to its maximum strength, and finally dying away again as slowly as it rose. The copper wire conveying the current was insulated from the sea-water by an envelope, usually of gutta-percha. Now the electricity sent into this wire induces electricity of an opposite kind to itself in the sea-water outside, and the attraction set up between these two kinds 'holds back' the current in the wire, and retards its passage to the receiving station.
With a receiving instrument set to indicate a particular strength of current, the rate of signalling would be very slow on long cables compared to land lines; so a different form of instrument was required for cable work. This fact stood greatly in the way of early cable enterprise. Thomson first solved the difficulty by his invention of the 'mirror galvanometer'. The merit of this receiving instrument is, that it indicates with extreme sensibility all the variations of the current in the cable, so that, instead of having to wait until each signal wave sent into the cable has travelled to the receiving end before sending another, a series of waves may be sent after each other in rapid succession. These waves, encroaching upon each other, will coalesce at their bases; but if the crests remain separate, the delicate decipherer at the other end will take cognisance of them and make them known to the eye as the distinct signals of the message.
The mirror galvanometer is at once beautifully simple and exquisitely scientific. It consists of a long fine coil of silk-covered copper wire, and in the heart of the coil, within a little air-chamber, a small round mirror, with four tiny magnets cemented to its back, is hung, by a single fibre of floss silk no thicker than a spider's line. The mirror is of film glass silvered, the magnets of hair-spring, and both together sometimes weigh only one-tenth of a grain. A beam of light is thrown from a lamp upon the mirror, and reflected by it upon a white screen or scale a few feet distant, where it forms a bright spot of light. When there is no current on the instrument, the spot of light remains stationary at the zero position on the screen; but the instant a current traverses the long wire of the coil, the suspended magnets twist themselves horizontally out of their former position, the mirror is of course inclined with them, and the beam of light is deflected along the screen to one side or the other, according to the nature of the current. If a positive current gives a deflection to the right of zero, a negative current will give a deflection to the left of zero, and vice versa. The air in the little chamber surrounding the mirror is compressed at will, so as to act like a cushion, and 'deaden' the movements of the mirror. The needle is thus prevented from idly swinging about at each deflection, and the separate signals are rendered abrupt and 'dead beat,' as it is called. At a receiving station the current coming in from the cable has simply to be passed through the coil of the 'speaker' before it is sent into the ground, and the wandering light spot on the screen faithfully represents all its variations to the clerk, who, looking on, interprets these, and cries out the message word by word. The small weight of the mirror and magnets which form the moving part of this instrument, and the range to which the minute motions of the mirror can be magnified on the screen by the reflected beam of light, which acts as a long impalpable hand or pointer, render the mirror galvanometer marvellously sensitive to the current, especially when compared with other forms of receiving instruments. Messages could be sent from the UK to the USA through one Atlantic cable and back again through another, and there received on the mirror galvanometer, the electric current used being that from a toy battery made out of a lady's silver thimble, a grain of zinc, and a drop of acidulated water.
The practical advantage of this extreme delicacy is that the signal waves of the current may follow each other so closely as almost entirely to coalesce, leaving only a very slight rise and fall of their crests, like ripples on the surface of a flowing stream, and yet the light spot will respond to each. The main flow of the current will of course shift the zero of the spot, but over and above this change of place the spot will follow the momentary fluctuations of the current which form the individual signals of the message. What with this shifting of the zero and the very slight rise and fall in the current produced by rapid signalling, the ordinary land line instruments are quite unserviceable for work upon long cables. The mirror instrument has one drawback, however: it does not 'record' the message. There is a great practical advantage in a receiving instrument which records its messages; errors are avoided and time saved. For this purpose, Thomson invented the siphon recorder, his second important contribution to the province of practical telegraphy. He aimed at giving a graphic representation of the varying strength of the current, just as the mirror galvanometer gives a visual one. The difficulty of producing such a recorder was, as he himself says, due to a difficulty in obtaining marks from a very light body in rapid motion, without impeding that motion. The moving body must be quite free to follow the undulations of the current, and at the same time must record its motions by some indelible mark. As early as 1859, Sir William sent out to the Red Sea cable a piece of apparatus with this intent. The marker consisted of a light platinum wire, constantly emitting sparks from a Rhumkorff coil, so as to perforate a line on a strip of moving paper; and it was so connected to the movable needle of a species of galvanometer as to imitate the motions of the needle. But before it reached the Red Sea the cable had broken down, and the instrument was returned dismantled, to be superseded at length by the siphon recorder, in which the marking point is a fine glass siphon emitting ink, and the moving body a light coil of wire hung between the poles of a magnet.
The principle of the siphon recorder is exactly the inverse of the mirror galvanometer. In the latter we have a small magnet suspended in the centre of a large coil of wire--the wire enclosing the magnet, which is free to rotate round its own axis. In the former we have a small coil suspended between the poles of a large magnet--the magnet enclosing the coil, which is also free to rotate round its own axis. When a current passes through this coil, so suspended in the highly magnetic space between the poles of the magnet, the coil itself experiences a mechanical force, causing it to take up a particular position, which varies with the nature of the current, and the siphon which is attached to it faithfully figures its motion on the running paper. The point of the siphon does not touch the paper, although it is very close. It would impede the motion of the coil if it did. But the 'capillary attraction' of so fine a tube will not permit the ink to flow freely of itself, so the inventor, true to his instincts, again called in the aid of electricity, and electrified the ink. The siphon and reservoir are together supported by an EBONITE bracket, separate from the rest of the instrument, and insulated from it. The ink may, therefore, be electrified to an exalted state, or high potential while the body of the instrument, including the paper and metal writing-tablet, are in connection with the earth, and at low potential, or none at all, for the potential of the earth is in general taken as zero. The tendency of a charged body is to move from a place of higher to a place of lower potential, and consequently the ink tends to flow downwards to the writing-tablet. The only avenue of escape for it is by the fine glass siphon, and through this it rushes accordingly and discharges itself in a rain upon the paper. The natural repulsion between its like electrified particles causes the shower to issue in spray. As the paper moves over the pulleys a delicate hair line is marked, straight when the siphon is stationary, but curved when the siphon is pulled from side to side by the oscillations of the signal coil.
The mouse-mill supplies both for the electricity which is used to electrify the ink and for the motive power which drives the paper. This unique and interesting little motor owes its somewhat epigrammatic title to the resemblance of the drum to one of those sparred wheels turned by white mice, and to the amusing fact of its capacity for performing work having been originally computed in terms of a 'mouse-power.' The mill is turned by a stream of electricity flowing from the battery above described, and is, in fact, an electro-magnetic engine worked by the current. The alphabet of signals employed is the 'Morse code'. The speed of signalling by the siphon recorder is se regulated by the length of cable through which it is worked. The instrument itself is capable of a wide range of speed. The best operators cannot send over thirty-five words per minute by hand, but a hundred and twenty words or more per minute can be transmitted by an automatic sender, and the recorder has been found on land lines and short cables to write off the message at this incredible speed. When we consider that every word is, on the average, composed of fifteen separate waves, we may better appreciate the rapidity with which the siphon can move. On an ordinary cable of about a thousand miles long, the working speed is about twenty words per minute.
To introduce his apparatus for signalling on long submarine cables, Sir William Thomson entered into a partnership with C. F. Varley, who first applied condensers to sharpen the signals, and Professor Fleeming Jenkin, of Edinburgh University. In conjunction with the latter, he also devised an "automatic curb sender," or key, for sending messages on a cable, as the well-known Wheatstone transmitter sends them on a land line. In both instruments the signals are sent by means of a perforated ribbon of paper; but the cable sender was the more complicated, because the cable signals are formed by both positive and negative currents, and not merely by a single current, whether positive or negative. Moreover, to curb the prolongation of the signals due to induction, each signal was made by two opposite currents in succession--a positive followed by a negative, or a negative followed by a positive, as the case might be. The after-current had the effect of curbing its precursor. This self-acting cable key was brought out in 1876, and tried on the lines of the Eastern Telegraph Company.
Sir William Thomson took part in the laying of the French Atlantic cable of 1869, and with Professor Jenkin was engineer of the Western and Brazilian and Platino-Brazilian cables. He was present at the laying of the Para to Pernambuco section of the Brazilian coast cables in 1873, and introduced his method of deep-sea sounding, in which a steel pianoforte wire replaces the ordinary land line. The wire glides so easily to the bottom that 'flying soundings' can be taken while the ship is going at full speed. A pressure-gauge to register the depth of the sinker was added by Sir William. About the same time he revived the Sumner method of finding a ship's place at sea, and calculated a set of tables for its ready application. His most important aid to the mariner is, however, the adjustable compass, which he brought out soon afterwards. It is a great improvement on the older instrument, being steadier, less hampered by friction, and the deviation due to the ship's own magnetism can be corrected by movable masses of iron at the binnacle.
Thomson did more than any other electrician up to his time to
introduce accurate methods and apparatus for measuring electricity. As early
as 1845 he pointed out that the experimental results of William Snow Harris
were in accordance with the laws of Coulomb. In the Memoirs of the Roman
Academy of Sciences for 1857 he published a description of his new divided
ring electrometer, based on the old electroscope of Bohnenberger and he introduced
a chain or series of effective instruments, including the quadrant electrometer,
which cover the entire field of electrostatic measurement. His delicate mirror
galvanometer was the forerunner of a later circle of equally precise apparatus
for the measurement of current or dynamic electricity.