January 3, 1908] 



SCIENCE 



work was the invention of the absolute 

 scale of measurement of temperature, 

 which is independent of the properties of 

 any thermometrie substance such as 

 mercury or air. By a fortunate accident 

 this scale (or one of the two proposed), 

 coincides nearly with that of a thermometer 

 using one of the more permanent gases like 

 hydrogen or nitrogen. The question of 

 how nearly it coincides could be decided 

 only by experiment, and these experiments 

 were carried out from 1852 to 1862 by 

 Thomson and Joule in collaboration, the 

 most important result obtained being that 

 on being forced through a porous plug all 

 gases except hydrogen were slightly cooled, 

 this cooling being shown to be due to the 

 slight attraction of the molecules of the gas 

 for each other, in spite of the tendency of 

 the gas to expand on account of the motion 

 of the molecules. It is probably by these 

 researches that Thomson as an experi- 

 mental physicist will be chiefly remem- 

 bered, for they furnish us, by the Joule- 

 Thomson effect, with our only means of re- 

 ducing the indications of an actual gas 

 thermometer to the absolute scale. 



We now come to a new subject, and the 

 one which made Thomson famous in the 

 eyes of the public, and which eventually 

 procured him his knighthood. At the be- 

 ginning of the agitation of the project of 

 the Atlantic telegraph cable, Thomson 

 plunging with enthusiasm directly into the 

 heart of the matter, took up the mathe- 

 matical question of the mode of propaga- 

 tion of signals in a telegraph line laid 

 under water. To this he again applied his 

 favorite Fourier mathematics, and in 1855 

 he communicated to the Royal Society a 

 paper in which the theory was completely 

 worked out, in which it was shown that the 

 current is propagated exactly as heat is 

 conducted, and that instead of being propa- 

 gated with a definite velocity, like sound, 

 so that a short signal would arrive, pass 



over and ceas«, the current would arrive 

 gradually, increase to a maximum, and die 

 away, always leaving an undesirable 

 residue to trouble the next signal. The 

 longer the cable the longer would it take 

 for the current to rise to its maximum, but 

 not in proportion. The vital question was, 

 how long would it take, and how much 

 current could be got through, and this he 

 solved in the most convincing fashion, with 

 the announcement of the possibility of the 

 prediction of the action of one cable by 

 the behavior of another. If K is the 

 capacity per unit of length, R the cor- 

 responding resistance, the time at which a 

 signal reaches its maximum value at a dis- 

 tance d away is proportional to the product 

 KBd". This is the famous fi^E-law, and 

 then follows the remarkable prediction, 

 "We may be sure beforehand that the 

 American telegraph will succeed, with a 

 batteiy sufficient to give a sensible cur- 

 rent at the remote end, when kept long 

 enough in action, but the time required for 

 each deflection will be sixteen times as long 

 as would be with a wire a quarter of the 

 length, such, for instance, as the French 

 submarine telegraph to Sardinia and 

 Africa." The mastery of the principles 

 of the telegraph thus shown led to the ap- 

 pointment of Professor Thomson as elec- 

 trician of the first cable laid in 1858, a 

 position which he held many times for later 

 cables. Not content with showing the con- 

 ditions necessary for success of working, 

 Thomson had invented an instrument to 

 make possible the reception of the weak 

 signals to be transmitted, and his mirror 

 galvanometer was ready when the shore 

 end of the cable was laid. The important 

 principle of this galvanometer was not 

 merely the long weightless index consist- 

 ing of a beam of light, the mirror principle 

 having been invented by Poggendorf, but 

 the reduction of the moving magnet to a 

 very small light affair weighing less than 



