September 21, 1906.] 



SCIENCE. 



359 



During the past twenty-five years much 

 thought has been devoted to the accurate 

 determination of certain physical con- 

 stants. 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 de- 

 fine it, the 'thermal equivalent of energy.' 

 "When Lord Kelvin addressed you in 1881, 

 I think it probable that he would have in- 

 dicated the value obtained by Joule — viz., 

 772.6 foot-pounds— at Manchester, as the 

 quantity of work required to raise the tem- 

 perature of one pound of water through 

 1° F. at 62° F. It is true that the results 

 of Rowland's classical investigation were 

 published in 1880 and 1881, but the dis- 

 crepancy between his conclusions and those 

 of Regnault regarding the change in the 

 specific heat of water at temperatures be- 

 tween 0° C. and 30° C. introduced an ele- 

 ment of uncertainty. 



As a consequence of this discrepancy 

 much experimental work on the subject has 

 been performed in the last quarter of a 

 century, and I think it may be said with- 

 out hesitation that the value of this impor- 

 tant constant is now ascertained with an 

 accuracy of about one part in 2,000. The 

 amount of labor which has been employed 

 in the determination of this thermal con- 

 stant is extraordinary, and, as I have 

 pointed out elsewhere, it well illustrates 

 the cosmopolitan character of scientific in- 

 vestigation. 



I have given reasons (Griffiths, 'The 

 Thermal Measurement of Energy') for 

 specially selecting for consideration the 

 determinations 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 Bal- 

 timore, Pisa, Zurich, Montreal, Manchester 

 and Cambridge have all contributed to the 

 solution of the problem, and we may now 



with some certainty say that 777.7 foot- 

 pounds at Greenwich are very closely the 

 equivalent of the amount of heat required 

 to raise one pound of water through 1° 

 on the hydrogen scale at 63.5° F. 



It may possibly appear that the result 

 just quoted is a somewhat poor return for 

 the expenditure of so much thought and 

 labor. I would call attention, therefore, 

 to the fact that the value of this equivalent 

 is dependent on the measurements of many 

 other natural constants; hence any agree- 

 ment 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 determina- 

 tions of mass, of change of temperature, 

 and of electrical resistance and current. 

 Certain discrepancies have led to the dis- 

 covery of a hitherto unsuspected 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 ven- 

 tured upon a prophecy that the value as- 

 signed 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 po- 

 tential difference 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 Phys- 

 ical Laboratory. 



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 re- 

 duced, they point to the conclusion that 



