Comparisons of Mercury and Platinum Thermometers. 19 



From the observed resistance of the platinum thermometer 

 and the corresponding temperature of the steam, the re- 

 sistance R ; at 100° C. was deduced as follows : — 



For example, from the first observation in the following 

 table (VII.) we have 



f Resistance R = 358*053 mean box-units (Box No. 7). 

 \ Red. barometric ht. = 755*19 mm. 



Corresponding temperature, t, of steam, obtained from 

 Broch's tables = 99°*822. 



The correction, dt/dp, to be applied to the air temperature, t, 

 of the boiling-point of water to reduce to the boiling-point at 

 760 mm. of mercury at 0° C, latitude 45°, sea-level, is there- 

 fore 0°*0368 per mm. of mercury. 



To find the corresponding correction to be applied to the 

 platinum temperature we have 



differentiating 



d.pt d.pt dt .n.ofi jj2/— 1001 \ 



— -j — — — 7 r- = *0ob8-< l — o ., n .,,,,, > • 



dp dt dp L 10000 J 



"We may assume an approximate value of 5=1*50 (from a 

 previous knowledge of the constants of platinum thermo- 

 meters) as sufficiently accurate for the purpose of this 

 reduction. 



If it happens that the values of 8 assumed differ greatly 

 from the final value obtained from the standardization, it may 

 be necessary to substitute this new value of S ; we have then 



— f- = 0°*0362 per mm. of mercury. 

 dp 



From the equation defining platinum temperature, 



i" = S o XlO0, 



we have 



R 1 = R + ^(R-R ). 



Reducing £=99°*822 to the corresponding temperature on 



the platinum scale, we have ^ = 99°*826. 



Hence 



nCD ,~i 1/1A (358*053-258*471) 

 R^ 258-4,1+ 100* wm ~ -I 



= 358*227 mean box-units. 



02 



