428 Mr. H. M. Tory on the 



nates, the differences P^— Pi , the difference-curve belonging 

 to each wire being denoted by the value of its d. It will be 

 seen that the difference-curves are all nearly straight, between 

 the limits o£ observation. It follows that a linear relation, 

 involving only two unknown quantities, will express the rela- 

 tion of any one of them to the standard, thus : — 



pt — pt f = a pt + b, 

 which may be written, if a is small, 



pt=pt' + apt' + b (4) 



By observing therefore the differences between the platinum 

 temperatures of the wires for two points, such as the S.B.P. 

 and the Ag F.P., the constants in formula (4) can be found 

 for each wire. 



Table I. 



Values of Constants a and b in the Linear Formula (4), for 

 each of the Impure Wires as compared with the Standard. 



No. of 

 Wire. 



Difference- 

 Coefficient,. 



Fundamental 

 Coefficient. 



Coefficient 

 a. 



Coefficient 

 b. 



1 



1595 



•003164 



+ •0103 



o 



-314 



2 



1-670 



•002689 



+ •0124 



-2 65 



3 



1645 



•003216 



+ •0142 



-3-80 



4 



1-530 



•002340 



-0106 



+5-73 



Since the differences are so small between the different 

 wires, the platinum temperatures observed with each wire 

 can be easily reduced to the scale of the standard wire by the 

 linear formula (4), and then corrected by the standard 

 difference-curve to the scale of the gas-thermometer. The 

 results thus obtained would agree within the limits of the 

 present series of observations with the Standard Scale recently 

 proposed by Callendar (Phil. Mag. Dec. 1899), which is based 

 on the application of the difference-formula (2) to the pure 

 standard wire. In this paper, it is pointed out that in all 

 probability the value there given for the melting-point of 

 silver, viz. 960° C, is correct to within 1° or 2°, and it is 

 suggested that a standard platinum wire be adopted, any other 

 wire being referred to it by determining the melting-point 

 of silver or gold, and the boiling-point of sulphur. To 

 the evidence there adduced might be added the redeter- 

 mination of the melting-points of gold and silver, by 



