PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 207 



this quantity of heat could be taken as equal to one water-gramme-degree 

 was for the time being left open. 



This resolution has made it incumbent on the Committee to consider 

 carefully — 



1. The relation between the results of measurements of intervals of 

 temperature by accepted methods and the absolute scale ; 



2. The specific heat of water in terms of the erg and its variation with 

 temperature. 



With regard to the first point there appears to be no reason to doubt 

 that the scale of a constant-volume hydrogen-thermometer is very nearly 

 identical with the absolute scale. ' The Committee have therefore decided 

 to recognise the standard hydrogen-thermometer of the Bureau Inter- 

 national des Poids et Mesures as representing, nearly enough for present 

 purposes, the absolute scale. This convention has at least the advantage 

 of giving a definite meaning to statements of the numerical value of 

 intervals of temperature within any range for which comparison with the 

 hydrogen-thermometer is practicable. If future investigation should show 

 that it is inaccurate to any appreciable extent, corresponding corrections 

 can be applied when necessary. 



Experience of the use of the platinum resistance-thermometer in various 

 hands encourages the hope that it will afford a convenient and trust- 

 worthy working method of referring the indications of mercury- or other 

 thermometers to those of the standard hydrogen-thermometer. The Com- 

 mittee have consequently much satisfaction in learning that Dr. C. A. 

 Harker, formerly of Owens College, is at this moment carrying out at 

 Sevres, on behalf of the Committee of the Kew Observatory, and with the 

 concurrence of the Director of the Laboratories of the Bureau Inter- 

 national, a direct comparison of platinum thermometers belonging to the 

 Kew Observatory with the standard hydrogen-thermometer of the 

 Bimreau. 



As to the dynamical value of the specific heat of water — in other words 

 the mechanical equivalent of heat — it was pointed out by Professor 

 SSchuster and Mr. Gannon in 1894 ^ that the results of the best determi- 

 nations by direct mechanical methods agree among themselves much more 

 •closely than they do with those that are founded upon electrical measure- 

 ments of the energy expended, although these in turn are in good agree- 

 ment among themselves. Additional significance is given to this remark 

 by the comparison of those determinations which, by extending over an 

 appreciable range of temperature, indicate the rate of variation of the 

 specific heat of water. Of such determinations there is one of each kind, 

 that of Professor Rowland by the mechanical method, and that of Mr. 

 E. H. Griffiths by the electrical method. The results of the former of these 

 liave recently undergone an elaborate revision at the hands of one of 

 Professor Rowland's pupils, Mr. W. S. Day,^ who has compared the 

 three principal thermometers employed in the experiments with the 

 ■Sevres hydrogen-standard by means of three Tonnelot thermometers 

 which had been compared at the Bureau with the hydrogen-standard. 

 Messrs. C W. Waidner and F. Mallory* have also compared two of 



' See Appendix No. 1 to this Eeport. 



^ Phil. Tranx., vol. clxxxvi., p. 462 ; Proc. Roy. Soc, vol. Ivii., p. 31. 

 ' Johns Hopkins University Circulars, pp. 44, 45 (June 1897) ; also Phil. Mag., 

 xliv. K9-172. 



* Ibid., pp. 42, 43 (June 1897) ; Phil Mag., xliv. 165-169. 



