NATURE 



137 



THURSDAY, DECEMBER 21, 1871 



THE COPLEY MEDALIST OF 1870 



' I ■'HIRTY years ago Electro-magnetism was looked to 

 J- as a motive power which might possibly coTipete 

 with steam. In centres of industry, such as Manchester, 

 attempts to investigate and apply this power were 

 numerou?, as shown by the scientific literature of the time. 

 Among others Mr. James Prescot Joule, a resident of 

 Manchester, took up the subject, and in a series of papers 

 published in Sturgeon's "Annals of Electricity" between 

 1839 and 1841, described various attempts at the con- 

 struction and perfection of electro-magnetic engines. The 

 spirit in which Mr. Joule pursued these inquiries is re- 

 vealed in the following extract : " I am particularly 

 anxious," he says, '■ to communicate any new arrangement 

 in order, if possible, to forestal the monopolising designs 

 of those who seem to regard this most interesting subject 

 merely in the light of pecuniary speculation" He was 

 naturally led to investigate the laws of electro-magnetic 

 attractions, and in 1840 he announced the important 

 principle that the attractive force exerted by two electro- 

 magnets, or by an electro-magnet and a mass of annealed 

 iron, is directly proportional to the square of the strength 

 of the magnetising current ; while the attraction exerted 

 between an electro -magnet and the pole of a perma- 

 nent steel magnet varies simply as the strength of the 

 current. These investigations were conducted inde- 

 pendently of, though a little subsequently to, the celebrated 

 inquiries of Henry, Jacobi^ and Lenz and Jacobi on the 

 same subject. 



On the 17th of December, 1840, Mr. Joule communi- 

 cated to the Royal Society a paper on the production of 

 heat by Voltaic electricity ; in which he announced the law 

 that the calorific effects of equal quantities of transmitted 

 electricity are proportional to the resistance overcome by 

 the current, whatever may be the length, thickness, shape, 

 or character of the metal which closes the circuit ; and 

 also proportional to the square of the quantity of trans- 

 mitted electricity. This is a law of primary importance. In 

 another paper, presented to but declined by the Royal 

 Society, he confirmed this law by new experiments, and 

 materially extended it. He also executed experiments on 

 the heat consequent on the passage of Voltaic electricity 

 through electrolytes, and found in all cases that the heat 

 evolved by the proper action of any Voltaic current is 

 proportional to the square of the intensity of that current 

 multiplied by the resistance to conduction which 

 it experiences. From this law he deduced a number 

 of conclusions of the highest importance to electro- 

 chemistry. 



It was during these inquiries, which are marked 

 throughout by rare sagacity and originality, that the great 

 idea of establishing quantitative relations between Mecha- 

 nical Energy and Heat arose and assumed definite form 

 in his mind. In 1843 Mr. Joule read before the meeting 

 of the British Association at Cork a paper " On the Calo- 

 rific Effects of Magneto-Electricity and on the Mechanical 

 Value of Heat." Even at the present day this memoir is 

 tough reading, and at the time it was written it must 



VOL. V. 



have appeared hopelessly entangled. This I should think 

 was the reason why Prof Faraday advised Mr. Joule not 

 to submit the paper to the Royal Society. But its drift 

 and results are summed up in these memorable words by 

 its author, written some time subsequently : " In that 

 paper it was demonstrated experimentally that the mecha- 

 nical power e.xerted in turning a magneto electric machine 

 is converted into the heat evolved by the passage of the 

 currents of induction through its coils, and on the other 

 hand, that the motive power of the electro-magnetic 

 engine is obtained at the expense of the heat due to the 

 chemical reaction of the battery by which it is worked.""' 

 It is needless to dwell upon the weight and importance of 

 this statement. 



Considering the imperfections incidental to a first 

 determination, it is not surprising that the " mechanical 

 values of heat," deduced from the different series of ex- 

 periments published in 1843, varied somewhat widely 

 from each other. The lowest limit was 587, and the highest 

 1,026 foot-pounds for 1° F. of temperature. 



One noteworthy result of his inquiries, which was 

 pointed out at the time by Mr. Joule, had reference to the 

 exceedingly small fraction of the heat which is actually 

 converted into useful effect in the steatn-engine. The 

 thoughts of the celebrated Julius Robert Mayer, who was 

 then engaged in Germany upon the same question, had 

 moved independently in the same groove ; but to his 

 labours due reference will doubtless be made on a future 

 occasion. In the memoir now referred to Mr. Joule eIso 

 announced that he had proved heat to be evolved during 

 the passage of water through narrow tubes ; and he 

 deduced from these experiments an equivalent of 770 

 foot-pounds, a figure remarkably near to the one nosv 

 accepted. A detached statement regarding the origin 

 and convertibility of animal heat strikingly illustrates the 

 penetration of Mr. Joule and his mastery of principles at 

 the period now referred to. A friend had mentioned to 

 him Haller's hypothesis, that animal heat might arise 

 from the friction of the blood in the veins and arteries. 

 " It is unquestionable," writes Mr. Joule, " that heat is 

 produced by such friction, but it must be understood that 

 the mechanical force expended in the friction is a part of 

 the force of affinity which causes the venous blood to 

 unite with oxygen, so that the whole heat of the system 

 must still be referred to the chemical changes. But if 

 the animal were engaged in turning a piece of machinery, 

 or in ascending a mountain, I apprehend that in pro- 

 portion to the muscular effort put forth for the purpose, a 

 diminution of the heat evolved in the system by a given 

 chemical action would be experienced." The italics in 

 this memorable passage, written it is to be remembered 

 in 1843, are Mr. Joule's own. 



The concluding paragraph of this British Association 

 paper equally illustrates his insight and precision regard- 

 ing the nature of chemical and latent heat. " I had," he 

 writes, "endeavoured to prove that when two atoms com- 

 bine together, the heat evolved is exactly that which 

 would have been evolved by the electrical current due to 

 the chemical action taking place, and is therefore pro- 

 portional to the intensity of the chemical force causing the 

 atoms to combine. I now venture to state more explicitly, 

 that it is not precisely the attraction of affinity, but rather the 



* Phil. M.ig. May 1845. 



