476 TRANSACTIONS OF SECTION A. 



specific heat, &c., with a clear indication of what is believed to be the prohable 

 accuracy of such determination. 



During the past twenty-five years much thought has been devoted to the 

 accurate determination of certain physical constants. This is very apparent in the 

 case of one of the most important — namely, that commonly termed the * mechanical 

 equivalent of heat,' or, as I prefer to define it, the 'thermal equivalent of energy.' 

 When Lord Kelvin addressed you in 1882, I think it probable that he would have 

 indicated the value obtained by Joule — viz., 772'6 foot-pounds — at Manchester, 

 as the quantity of work required to raise the temperature of one pound of water 

 through 1° r. at 62° F. It is true that the results of Rowland's classical investi- 

 gation were published in 1880 and 1881, but the discrepancy between his conclu- 

 sions and those of Eegnault regarding the change in the specific heat of water at 

 temperatures between 0° C. and 30° C. introduced an element of uucertaintj'. 



As a consequence of this discrepancy much experimental work ou the subject 

 has been performed in the last quarter of a century, and I think it may be said 

 without hesitation that the value of this important constant is now ascertained 

 with an accuracy of about one part in 2,000. The amount of labour which 

 has been employed in the determination of this thermal constant is extraordinary, 

 and, as I have pointed out elsewhere, it well illustrates the cosmopolitan 

 character of scientific investigation, 



I have given reasons ' for specially selecting for consideration the determina- 

 tions of Rowland, of Bartoli and Stracciati, of Ludin, of Callendar and Barnes, of 

 Schuster and Gannon, and I have ventured to add my own. Thus Baltimore, 

 Pisa, Zurich, Montreal, Manchester, and Cambridge have all contributed to the 

 solution of the problem, and we may now with some certainty say that777"7 foot- 

 pounds at Greenwich are very closely the equivalent of the amount of heat required 

 to raise 1 lb. of water through 1° ou the hydrogen scale at G3°"5 F. 



It may possibly appear that the result just quoted is a somewhat poor return 

 for the expenditure of so much thought and labour. I would call attention, there- 

 fore, to the fact that the value of this equivalent is dependent on the measurements 

 of many other natural constants ; hence any agreement between the results 

 obtained by the observations of Rowland and some of the other observers I have 

 mentioned would only be possible in the absence of errors of appreciable magnitude 

 in the determinations of mass, of change of temperature, and of electrical resistance 

 and current. Certain discrepancies have led to the discovery of hitherto unsus- 

 pected cause of inaccuracy, especially in the determination of temperature, and 

 thus the inquiry has rendered valuable service in many branches of physical 

 inquiry. 



For example, so far back as 1893 I ventured upon a prophecy that the value 

 assigned to the E.M.F. of a Clarke's cell was somewhat too high, and that it 

 was possible that 1-4328 represents more truly the potential diftereuce of a 

 Clarke's cell at 15° C. than the ordinarily accepted value of 1"4342. In the report 

 of the Electrical Standards Committee for 1897 will be found a discussion of this 

 matter, and one of the consequences of the deliberations of that Committee is 

 to be seen in the ampere balance now standing in the National Physical Labora- 

 tory. 



The results of the observations obtained by this instrument will, I believe, 

 shortly be published by Professor Ayrton and Mr. Mather, but I am at liberty to 

 state that, so far as the observations have been reduced, they point to the conclusion 

 that the prophecy to which I have referred is closelj fulfilled. We may say, there- 

 fore, with some confidence that the values of those units which form the basis of our 

 system of electrical measurement are not only practically determined with a high 

 degree of accuracy, but that also our measurements of temperature and of energy 

 are placed on a satisfactory footing. 



The last few years have been fruitful ia revelations which not only profoundly 

 affect the views of students of science, but also are of such a nature as to catch 

 the eye of the public. In some cases the applications of these discoveries to the 

 purposes of mankind have been evident and immediate. Every well-equipped 



• GriflSths, The Thermal Mcaswement of Energy. 



