LIFE AND WORK OF LORD KELVIN THOMPSON. 759 



myself," he said, " what seems the shortest and surest way to reach 

 the philosophy of measurement- — an understanding of what we mean 

 by measurement, and which is essential to tlie intelligent practice of 

 the mere art of measuring— is to cut off all connection with the earth." 

 And so he imagined a traveler with no watch or tuning fork or meas- 

 uring rod wandering through the universe trying to recover his centi- 

 meter of length and his second of time and reconstructing thereupon 

 his units and standards from the wave length of the yellow light of 

 sodium and the value of v the velocity of light from experiments on 

 the oscillations in the discharge of a Leyden jar ! Some of us in this 

 very room remember how we listened amazed to this characteristic 

 and bewildering excursus. 



Among the activities of these fruitful years was a long research on 

 the electrodynamic qualities of metals — thermoelectric, thermoelastic, 

 and thermomagnetic. These formed the subject of his Bakerian lec- 

 ture of 185G, which occupies no fewer than 118 pages of the reprinted 

 Mathematical and Physical Papers. He worked hard also at the 

 mathematical theory of magnetism. Faraday's work on diamagnet- 

 ism had appeared while Thomson was a student at Cambridge. It 

 established the fact that magnetic forces were not mere actions at a 

 distance between supposed poles, but actions dependent on the sur- 

 rounding medium; and Thomson set himself to investigate the matter 

 mathematically. Faraday and Fourier had been the heroes of Thom- 

 son's youthful enthusiasm; and, while the older mathematicians shook 

 their heads at Faraday's heretical notion of curved lines of force, 

 Thomson had, in 1849 and 1850, developed a new theory with all the 

 elegance of a mathematical disciple of Poisson and Laplace, discuss- 

 ing solenoidal and lamellar distributions by aid of the hydrodynamic 

 equation of continuity. To Thomson we owe the terms " permeabil- 

 ity " and " susceptibility," so familiar in the consideration of the mag- 

 netic properties of iron and steel. He continued to add to and revise 

 this work through the sixties and seventies. 



In 1859-60 Thomson was studying atmospheric electricity, writing 

 on it in Nichol's Cyclopedia and lecturing on it at the Royal Institu- 

 tion. For this study he invented the water-dropping collector, and 

 vastly improved the electrometer, which developed into the elaborate 

 forms of the quadrant instrument and other types described in the 

 B. A. report of 1867. During this work he discovered the fact that 

 the sudden charge or discharge of a condenser is accompanied b}'^ a 

 sound. He also measured electrostatically the electromotive force of 

 a Daniell cell, and investigated the potentials required to give sparks 

 of different lengths in the air. 



In the winter of 1860-61 Thomson met with a severe accident. He 

 fell on the ice when engaged at Largs in the pastime of curling and 

 88292— SM 1908 49 



