The Comparison of Thermometers. 357 



On the Comparison of Thermometers. By Thos. Ewan^ 

 Ph.D., B.Sc, and W. W. Haldane Gee, B.Sc, F.C.S. 



(^Received March 24.th^ i8gi.) 



The experiments described in the following paper were 

 undertaken with the object of finding a practical method 

 for standardizing platinum resistance thermometers and 

 comparing them with mercurial thermometers. 



The platinum thermometer, as suggested by Siemens,, 

 has been improved by Callendar,* who found that, if 

 the ratio 



\Rioo — R-o/ ' 



be taken as * temperature by platinum thermometer ' =pt 

 [where R^, R^, and Rioo are the resistances at t°, o^, and 

 100^ respectively], then the differences between the readings 

 of an air thermometer and those of the platinum ther- 

 mometer are represented by the equation 



,_^,=a|(_lj_ 



t 



100 



For the specimen of platinum wire used by Callendar 

 the constant ^ was found to be I'S?- This formula agreed 

 with the results within 1% through a range of 700 degrees. 



Griffithsf has confirmed these results, and has shown 

 that the ^ formula represents the differences between the 



•Phil. Trans. 1887. A. 



+ Brit. Ass., Leeds Meeting, 1890. {^Electrician^ Oct., 1890.) 

 Proc. Roy. Soc, June, 1890. 

 Callendar and Griffiths, Proc. Roy. Soc, Dec, 1890. 



