1898.] of temperature by means of Platinum Thermometry. 529 



Example 1. 



Pyr. 22. cf>= 99786, z =245771, 



S = 1-490, a' = 1085815, 



22 = 257-885, 5=7929-71, 



[a = 1012728]. 



Test : z ^-"' = 257-885, 



but R = 257-885. 



Example 2. 



Pyr. 25. </> = 99-206, * = 246*765, 

 8= 1-5135, a'= 1081240, 

 R = 257666, 5=7786-18, 



[a = 1-0070850]. 



Test: *<^ = 257-666, 



but ^0 = 257-666. 



I have calculated what would be the resistance of these 

 pyrometers at the melting point of gold (960°), using first 

 equation (3) of § 3, secondly the rule 



[«'(* + *)? 



R = a'(t + z)- 



8 



The exact agreement only proves of course that the arithmetic 

 and algebra has been correctly performed, but this it seems to me 

 is all that need be proved. The conclusion is that by using this 

 form of bridge the same temperature can be arrived at without 

 calculation as would have been arrived at by the 8 formula. 

 Whether this method of calculating the temperature is itself 

 correct is of course outside the object of this paper. 



In concluding this note it may be well to point out a curious 

 analogy between the action of the shunt and the action of the 

 impurities in the wire. It is regarded by many as not improbable 

 that in a wire of perfectly pure platinum the resistance might be 

 a linear function of the temperature, and then no shunt would be 

 required to give direct readings. If so we may regard the action 

 of the impurity in the wire as similar to a shunt in providing 

 an alternative path to the flow of electricity. It seems hardly 

 worth while to push the idea further in a paper whose object is 

 entirely practical. 



I am greatly indebted to Mr Griffiths and also to Messrs 

 Neville and Heycock for the assistance received from them. 



VOL. IX. PT. VIII. 42 



