Lord Kelvin: An account of his scientific life and work

by Andrew Gray

In some of the notices of Lord Kelvin that have appeared in the newspapers, the imagination of the writers has converted the Jacob rifle into one which Professor Thomson carried in the early years of the volunteer movement, as a member of a Glasgow corps. It is still used in the Natural Philosophy Department for the same experiment, and is a muzzle-loading rifle of large calibre, which throws an ounce bullet. It was invented by the well-known Indian sportsman, Colonel Jacob, for big-game shooting in India. Thomson held a commission as captain in the K (or University) Company of rifle volunteers, and so did not shoulder a rifle, except when he may have indulged in target practice.

The front bench students were always in a state of excitement, mingled in some cases perhaps with a little trepidation. For the target was very near them, and though danger was averted by placing a large wooden screen in front of the bob, to prevent splinters of the bullet from flying about in the event of its missing the target and striking the iron casing of the bob, there was a slight amount of nervousness as to what might happen. The rifle, loaded by McFarlane, who had weighed out the charge of powder (so many drams) from a prescription kept in a cavity of the stock, was placed on the table, and two rests, provided with V notches to receive the rifle, were placed in the proper position to enable a bull's eye to be obtained. Thomson generally produced a small box of cotton wool, and inserted a little in each of his ears to prevent injury to the tympanum from the report, and advised the spectators to do the same. Then, adjusting his eye-glass, he bent down, placed the rifle in position, and fired, and the solemn stillness with which the aiming and adjustments had been witnessed was succeeded by vociferous applause. The length of tape drawn out under a light spring was read off by McFarlane, who had already placed on the blackboard the formula for calculation of the velocity, with the factor by which the length of tape had to be multiplied to give the velocity in feet per second. Then, with the intimation that a question involving numerical calculation would be set on the subject, in the ensuing Monday morning examination paper, the lecture generally closed, or was rounded off with some further observations on angular (or, as Thomson always preferred to call it, moment of) momentum.

Long after in the course of a debate in the House of Lords on a proposal to make the use of the metric system of weights and measures compulsory, Lord Kelvin told their lordships how he had weighed out the powder to charge this rifle, and, mistaking the weights, had loaded the rifle with an amount of powder which would have been almost certain to burst the piece, but had happily paused before firing it off.

He often interrupted the course of a lecture with a denunciation of the British "no-system of weights and measures"—"insane," "brain-wasting," "dangerous," were among the mildest epithets he applied to it, and he would deeply sympathise with the student whose recollection of avoirdupois weight, troy weight, apothecaries' weight, etc., was somewhat hazy. The danger of the system consisted mainly in the fact that the apothecaries' dram is 60 grains, while the avoirdupois dram is 27¹⁄₃ grains. Thus so many drams of powder required to charge a rifle is a very much larger quantity when reckoned in apothecaries' drams than when reckoned in avoirdupois. As a rule he left the loading of the rifle, like all the other lecture-room experiments, to his assistants.

Another experiment which caused a great sensation was that known as the "dew-drop"! A funnel of brass, composed of a tube about 30 inches long and an inch wide, and a conical mouth about ten inches wide, had a piece of stout sheet India-rubber stretched, as tightly as it could be by hand, across its mouth, and made water-tight by a serving of twine and cement round the edge. A wire soldered round the outside of the lip gave a good hold for this serving and made all perfectly secure. On the plane surface of the sheet geometrical figures were drawn in ink, so that their distortion could be afterwards studied. The funnel was then hung by a strong support in an inverted position behind the table, and water poured gently into it from a rubber supply pipe connected with the water-main. As the water was allowed to accumulate—very slowly at first—the sheet of rubber gradually stretched and bulged out, at first to a flat lens-shape, and gradually more and more, till an immense water-drop had been formed, 15 or 18 inches in horizontal diameter, and of still greater vertical dimensions. The rubber film was now, at the place of greatest tension, quite thin and transparent, and its giving way was anticipated by the students with keen enjoyment. A large tub had been placed below to receive the water, but the deluge always extended over the whole floor space behind the table, and was greeted with rapturous applause.

Before the drop burst, and while it was forming, Thomson discoursed on surface tension, emphasising the essential difference between the tension in the rubber-film and the surface-film of a dewdrop, and pointing out how the geometrical figures had changed in shape. Then he would poke it with the pointer he held in his hand, and, turning to the class, as the mass quivered, remark, "The trembling of the dewdrop, gentlemen!"

Vibrations of elastic solids were illustrated in various ways, frequently by means of a symmetrical shape of calves'-foot jelly, at the top of which a coloured marble had been imbedded as a molecule, the motions of which could be followed. And then he would discourse on the Poisson-Navier theory of isotropic solids, and the impossibility of the fixed relation which that theory imposed between the modulus of rigidity and the modulus of compression; and refer with approval to the series of examples of "perfectly uniform, homogeneous, isotropic solids," which Stokes had shown could be obtained by making jellies of different degrees of stiffness. Another example, frequently adduced as indicating the falsity of the theory, was the entirely different behaviour of blocks of India-rubber and cork, under compression applied by a Bramah press. The cork diminished in thickness without spreading out laterally; the rubber, being very little compressible, bulged out all round as its thickness was diminished.

The lectures on acoustics, which came late in the course, were also exceedingly popular. Two French horns, with all their crooks and accessories, were displayed, and sometimes, to the great delight of the class, Thomson would essay to show how the pitch of a note could be modified by means of the keys, or by the hand inserted in the bell. The determination by the siren of the pitch of the notes of tuning-forks excited by a 'cello bow, and the tuning of a major third by sounding at the same time the perfect fifth of the lower note, were often exhibited, and commented on with acute remarks, of which it is a pity no statement was ever published.[25]

The closing lecture of the ordinary course was usually on light, and the subject which was generally the last to be taken up—for as the days lengthened in spring, it was possible sometimes to obtain sunlight for the experiments—was often relegated to the last day or two of the session. So after an hour's lecture Thomson would say, "As this is the last day of the session, I will go on for a little longer, after those who have to leave have gone to their classes." Then he would resume after ten o'clock, and go on to eleven, when another opportunity would be given for students to leave, and the lecture would be again resumed. Messengers would be sent from his house, where he was wanted for business of different sorts, to find out what had become of him, and the answer brought would be, hour after hour, "He is still lecturing." At last he would conclude about one o'clock, and gently thank the small and devoted band who had remained to the end, for their kind and prolonged attention.

In the course of his lectures Thomson continually called on his assistants for data of all kinds. In the busiest time of his life—the fifteen years from 1870 to 1885—he trusted to his assistants for the preparation of his class illustrations, and it was sometimes a little difficult to anticipate his wishes, for without careful rehearsal it is almost impossible to make sure that in an experimental lecture everything will go without a hitch. The digressions, generally most interesting and instructive, in which he frequently indulged, almost always rendered it necessary to bring some experiment before the class which had not been anticipated, and all kinds of things were kept in readiness, lest they should be wanted suddenly.