Comparison of Thermometers. 87 



On a Comparison of the Thermometers used by the late 

 Dr. Joule with the Standards of the Bureau Inter- 

 national des Poids et Mesures. By Arthur Schuster, 

 F.R.S. 



(Received January 22nd, 1894..) 



The mechanical equivalent of heat being defined as the 

 quantity of work necessary to raise unit mass of water at a 

 stated temperature through one degree, it is clear that its 

 value must depend on what we adopt as the unit of our 

 temperature scale. It is not sufficient to fix on the tempera- 

 ture of freezing or boiling water, and to call them o° and 

 100° respectively, for this leaves it still doubtful what we 

 should take to be 50° or 6o°. It has been usual for a long 

 time to take the expansion of mercury in a glass vessel 

 as our guide, and to fix the temperature intermediate 

 between o° and 100° by means of the mercury thermometer. 

 For many purposes this is sufficiently accurate, but the 

 irregular behaviour of such thermometers has led to some 

 difficulties which have only been recently surmounted. The 

 nature of the glass having a considerable influence on its 

 coefficient of expansion, it has become necessary carefully 

 to compare thermometers made of different kinds of glass, 

 and those who have followed the recent development of this 

 subject will know of the great progress which has been 

 made in this direction by the Technische Reichs Anstalt 

 and the Bureau International des Poids et Mesures. 



Since the late Dr. Joule concluded his classical researches, 

 several accurate re-determinations of the equivalent of heat 

 have been made, and in order to compare the results obtained 

 by different observers it seemed to me to be of great interest 

 to find the scale value of Joule's thermometers in terms of 



