482 Prof. A. Schuster on the Scale-Value of 



The Fundamental Points. 



As regards the thermometer D, Joule has supplied us with 

 the following information : — 



" The freezing-point of the standard D had risen from 13*3 

 divisions of its scale in 1844 to 15*14 in 1877. I think it 

 probable that the boiling-point of this thermometer, if kept 

 constantly at this temperature, would in the course of time 

 fall as much. The five careful determinations of this boiling- 

 point referred to 30 bar. and 60° are respectively 706, 706*4, 

 706, 705-9, and 706*15 -mean 706*09. Subtracting 1*84, 

 704*25 will be the probable ultimate reading, from which if 

 we take 15*14 we shall have 689*11 as the range between the 

 fixed points cleared from the effects of imperfect elasticity of 

 the glass. Mr. E. Hodgkinson has pointed out (Brit. Assoc. 

 Report, 1843, p. 23) that the ' set ' of imperfectly elastic 

 bodies is proportional to the square of the force applied, 

 therefore the effect of imperfect elasticity in the glass of the 

 thermometers will be insensible for the small ranges used in 

 the experiments, and the factor 3*3822 for reducing the 

 indications of D to those of A may be confidently relied on. 



" We have therefore 



1 80 



-0°*07723 



689*11x3*3822 



as the most probable value of one division of A. In my 

 former papers the number was taken as 0°*077214, which is 

 so near that I shall continue to use it, trusting by long- 

 continued observations of the fixed points to give it ultimately 

 greater accuracy, and also, by experiments above indicated, 

 to state it in terms of the absolute interval between these 

 points." (Phil. Trans. 1878, part ii.; Collected Works, vol.i. 

 p. 636.) 



It will be noticed that the actually observed difference 

 between the freezing- and boiling-point is 690*95 divisions, 

 but that Joule somewhat arbitrarily reduced this by 1*84 

 divisions, thus altering the fundamental interval by over a 

 quarter per cent. It seems curious that no one should have 

 directed his attention to this point, which to all appearance 

 causes an error in the scale-value of his thermometer, and 

 would make his equivalent come out too low by *0027 of its 

 own value. 



If we collect together the scale- values of the thermometer 

 A, given by Joule in different places, we find : — 



