Comparison of Platinum Thermometers. 



429 



M. Berthelot, who gives the two points as 962 c C. and 

 1064° 0. It seems to be a reasonable conclusion, therefore, 

 that from 0° up to the melting-point of gold, 1060°, at least, 

 a practical thermometric scale could be established, which 

 would at no point differ from the gas-thermometer scale by 

 more than 2° or 3°. It would follow that all observers 

 working with platinum pyrometers could, in a most certain 

 and ready manner, compare their results. The standard 

 sample would thus be defined as a wire with a definite sulphur 

 boiling-point, and a definite silver melting-point. In fact a 

 number of definite points could be defined up the scale, any 

 two of which could be used. 



I have not yet carefully studied the difference below 400° 

 or above 1000°, but hope to do so in the near future. It 

 would appear, however, from results obtained, that the linear 

 difference-curve commences to bend below 400°, meeting 

 the reference line at 100° C. Such a process of reduction is 

 hardly necessary, however, helow 500° C, as in any good 

 specimen of platinum wire the temperatures can be reduced 

 directly by the difference-formula, using the value of d deter- 

 mined from the S.B.P. 



7. Variation of Temperature- Coefficients. — Since the differ- 

 ences between the platinum temperatures of two wires 

 depend, on the rate of change of the temperature-coefficients 

 of the wires, the following table may not be uninteresting: — 



Table II. 

 Values of Temperature- Coefficients over Different Ranges. 



No. of 

 Wire. 



Difference 

 Coefficient. 



Funda- 

 mental 

 Coefficient, 

 c. 



Coefficient 

 0°-445°. 



Per- 

 cent, 

 of c. 



Coeffi- 

 cient 

 0°-960°. 



Per- 

 cent, 

 of c. 





S.B.P. 



AgF.P. 













1 



1-595 



1-561 



•003164 



002991 



946 



•002739 



866 



2 



1-670 



1-592 



•002689 



•002533 



94-7 



•002322 



86-4 



3 



1-645 



1-600 



•003216 



•003033 



94-3 



•002775 



86-3 



4 



1-530 



1-461 



•002340 



•002215 



946 



•002047 



87-4 



Standard 



1-500 



1-500 



•003892 1 



•003692 



94-8 



003391 



87-0 



It will be observed that the percentage diminution of the 

 coefficient of each of the wires is nearly the same over the 

 range 0° to 960° in terms of the fundamental coefficient. 



