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William Thomson, 1st Baron Kelvin

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This article is about the physicist; there was also an Archbishop of York of the same name.

William Thomson, 1st Baron Kelvin, (June 26, 1824December 17, 1907) was a mathematical physicist who did important work in thermodynamics. The SI base unit of temperature, the Kelvin, is named after him.


Transatlantic cable

Thomson became a man of public note in connection with the laying of the first transatlantic telegraph cable.

When a 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.

Thomson's 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.

Thomson's law was disputed in 1856 by 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.

This measurer of the current was far more sensitive than any which preceded it, enabling the detection of 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.

There were several unsuccessful attempts before the cable was finally laid by the SS Great Eastern.

Mirror galvanometer

Thomson invented the mirror galvanometer in response to the need for an instrument that could indicate with sensibility all the variations of the current in a long cable.

The mirror galvanometer consists of a long fine coil of silk-covered copper wire. In the heart of that coil, within a little air-chamber, a small round mirror is hung by a single fibre of floss silk, with four tiny magnets cemented to its back. 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 electric 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. At a receiving station the current coming in from the cable has simply to be passed through the coil 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.

Siphon recorder

The mirror galvanometer does not record the message. For this purpose, Thomson invented the siphon recorder, his second important contribution to the province of practical telegraphy.

The principle of the telegraph 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, to avoid impeding the motion of the coil.

The siphon and an ink reservoir are together supported by an ebonite bracket, separate from the rest of the instrument, and insulated from it. This separation permits the ink to be electrified to a high potential while the body of the instrument, including the paper and metal writing tablet, are grounded, and at low potential. 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 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.

To introduce his apparatus for signalling on long submarine cables, Thomson entered into a partnership with C. F. Varley, who first applied condensers to sharpen the signals, and Professor Fleeming Jenkin of the University of Edinburgh. In conjunction with the latter, he also devised an automatic curb sender (telegraph 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 aftercurrent 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.

Thomson took part in the laying of the French Atlantic submarine communications 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 Pará to Pernambuco section of the Brazilian coast cables in 1873.

Other activities and contributions

Thomson introduced a method of deep-sea sounding, in which a steel piano 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.

Sir William was an enthusiastic yachtsman. His interest in all things relating to the sea perhaps arose, or at any rate was fostered, by his experiences on the Agamemnon and the Great Eastern. Charles Babbage was among the first to suggest that a lighthouse might be made to signal a distinctive number by occultations of its light; but Sir William pointed out the merits of the Morse code for the purpose, and urged that the signals should consist of short and long flashes of the light to represent the dots and dashes.

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.

The age of the Earth

Lord Kelvin is also known for his (mistaken) estimate of the age of the Earth. As quoted above, he said "This earth, certainly a moderate number of millions of years ago, was a red-hot globe..."—this was on the assumption that the Earth's internal heat came from cooling from a red-hot state. This estimate was in conflict with the timescales estimated by contemporary geologists and evolutionists. Viewed with hindsight, his calculations were correct but his assumptions wrong (he overlooked radioactivity, which had not yet been discovered).

Statue of Lord Kelvin; Botanic Gardens,
Statue of Lord Kelvin; Botanic Gardens, Belfast

Personal life

Early years

Thomson was born in Belfast . His father, Dr James Thomson, a blacksmith, had educated himself at University of Glasgow while working as a teacher, and later became its chair of mathematics. William began his course there in his eleventh year, and was noted for his extraordinary speed in solving the problems of his father's class. After finishing at Glasgow he was sent to the higher mathematical school of Peterhouse, Cambridge. In 1845 he graduated as second wrangler, and won the Smith prize. This "consolation stakes" is regarded as a better test of originality than the tripos. The first, or senior, wrangler only needed a facility in applying well-known rules, and a readiness in writing. One of the examiners is said to have declared that he was unworthy to cut Thomson's pencils.


While at Cambridge, Thomson was active in sports and athletics. He won the Silver Sculls, and rowed in the winning boat of the Oxford and Cambridge Boat Race. He also took a lively interest in the classics, music, and literature; but the real love of his intellectual life was the pursuit of science. The study of mathematics, physics, and in particular, of electricity, had captivated his imagination. At seventeen, young Thomson had begun to conduct original research. The Cambridge Mathematical Journal of 1842 contains a paper by him—"On the uniform motion of heat in homogeneous solid bodies, and its connection with the mathematical theory of electricity". In this he demonstrated the identity of the laws governing the distribution of electric or magnetic force in general, with the laws governing the distribution of the lines of the motion of heat in certain special cases. The paper was followed by others on the mathematical theory of electricity; and in 1845 he gave the first mathematical development of Faraday's idea that electric induction takes place through an intervening medium, or "dielectric", and not by some incomprehensible "action at a distance". He also devised a hypothesis of electrical images, which became a powerful agent in solving problems of electrostatics, or the science which deals with the forces of electricity at rest. It was partly in response to his encouragement that Faraday undertook the research in September of 1845 that led to the discovery of the Faraday effect, which established that light and magnetic (and thus electric) phenomena were related.

On gaining a fellowship at his college, he spent some time in the laboratory of the celebrated Henri Victor Regnault, at Paris; but in 1846 he was appointed to the chair of natural philosophy in the University of Glasgow. At twenty-two he found himself wearing the gown of a learned professor in one of the oldest Universities in the country, and lecturing to the class of which he was a freshman but a few years before.

Religious beliefs

Thomson remained a firm believer in Christianity throughout his life, as evident in the following quotation from the annual meeting of the Christian Evidence Society , May 23, 1889:

"I have long felt that there was a general impression in the non-scientific world, that the scientific world believes Science has discovered ways of explaining all the facts of Nature without adopting any definite belief in a Creator. I have never doubted that that impression was utterly groundless. It seems to me that when a scientific man says—as it has been said from time to time—that there is no God, he does not express his own ideas clearly. He is, perhaps, struggling with difficulties; but when he says he does not believe in a creative power, I am convinced he does not faithfully express what is in his own mind, He does not fully express his own ideas. He is out of his depth.
"We are all out of our depth when we approach the subject of life. The scientific man, in looking at a piece of dead matter, thinking over the results of certain combinations which he can impose upon it, is himself a living miracle, proving that there is something beyond that mass of dead matter of which he is thinking. His very thought is in itself a contradiction to the idea that there is nothing in existence but dead matter. Science can do little positively towards the objects of this society. But it can do something, and that something is vital and fundamental. It is to show that what we see in the world of dead matter and of life around us is not a result of the fortuitous concourse of atoms.
"I may refer to that old, but never uninteresting subject of the miracles of geology. Physical science does something for us here. St. Peter speaks of scoffers who said that 'all things continue as they were from the beginning of the creation'; but the apostle affirms himself that 'all these things shall be dissolved'. It seems to me that even physical science absolutely demonstrates the scientific truth of these words. We feel that there is no possibility of things going on for ever as they have done for the last six thousand years. In science, as in morals and politics, there is absolutely no periodicity. One thing we may prophesy of the future for certain—it will be unlike the past. Everything is in a state of evolution and progress. The science of dead matter, which has been the principal subject of my thoughts during my life, is, I may say, strenuous on this point, that THE AGE OF THE EARTH IS DEFINITE. We do not say whether it is twenty million years or more, or less, but let me say it is not indefinite. And we can say very definitely that it is not an inconceivably great number of millions of years. Here, then, we are brought face to face with the most wonderful of all miracles, the commencement of life on this earth. This earth, certainly a moderate number of millions of years ago, was a red-hot globe; all scientific men of the present day agree that life came upon this earth somehow. If some form or some part of the life at present existing came to this earth, carried on some moss-grown stone perhaps broken away from mountains in other worlds; even if some part of the life had come in that way—for there is nothing too far-fetched in the idea, and probably some such action as that did take place, since meteors do come every day to the earth from other parts of the universe;—still, that does not in the slightest degree diminish the wonder, the tremendous miracle, we have in the commencement of life in this world."


In recognition of his achievements, he was in 1892 created Baron Kelvin. He was elected a Fellow of the Royal Society in 1851 (serving as its President, 18901895), was appointed Knight Grand Cross of the Victorian Order in 1896, and in 1902 became one of the first members of the Order of Merit as well as a Privy Counsellor. After his death he was buried in Westminster Abbey, London.


  • John Munro, Heroes of the Telegraph (London: Religious Tract Society, 1891)

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