THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JUNE 1895. 



XL VIII. On the Scale-Value of the late Dr. Joule 5 s 

 Thermometers. By Arthur Schuster, F.R.S.* 



[Plates V. & VI.] 



IN order to bring the results of Joule's researches on the 

 mechanical equivalent of heat into relation with more 

 modern experiments on the same subject, it is necessary to 

 determine the scale-value of Joule's thermometers in terms 

 of some easily reproducible standard. 



We possess already a comparison by Joule himself of his 

 thermometer with one used by Rowland, who has corrected 

 Joule's result to the scale of his own air-thermometer. 



Some doubt may still exist, however, as to the true scale- 

 value of these instruments, partly owing to the fact that we 

 have no information how the comparison between Joule's and 

 Rowland's thermometers was conducted, and partly because 

 we do not know to what degree of accuracy Rowland's air- 

 thermometer would agree with that of the Bureau International 

 des Poids et Mesures, which for the present must be con- 

 sidered as the standard. 



The historical importance of the instruments used by 

 Joule seemed to make it desirable therefore to subject them 

 to a more extended investigation. The request which I made 

 to Mr. B. A. Joule to allow me the use for a short time of his 

 late father's thermometers was met by a most ready compliance, 



* Oomrnunicated by the Author. 

 Phil Mag. S. 5. Vol. 39. No. 241. June 1895. 2 K 



