Platinum Thennomctry at Kcw Observatory. 49 



interval R* R directly, I pass to the consideration of pt g and , as 

 quantities of more immediate interest. The constancy of one of these 

 quantities is involved in that of the other ; but each has an interest of 

 its own. I have thought it desirable to give the results obtained from 

 formula (17) as well as those obtained by accepting the Callendar- 

 Griffiths formula (10). 



The thermometer called K 2 in 1896 is wholly different from that so 

 denominated in 1899. 



With the exception of the 1899 results in K 2 , and the 1897 results 

 in Kt, every mean in the table depends on at least four observations ; 

 in some instances eight were available. 



The existence of a gradual rise in the pt g of KI, with corresponding 

 fall in its 8, is placed I think beyond doubt. In the case of the other 

 thermometers there is no fluctuation which might not reasonably be 

 attributed to errors of observation or uncertainty in the pressure reduc- 

 tion formula. 



Differences between tJie TJiermometers, 



46. A difference between the values of RO or I in two thermometers 

 does not necessarily imply any difference between them as measurers 

 of temperature. The temperature pt is a ratio, and so should depend 

 solely on the law of increase of resistance with temperature. 



If the drawing of platinum wire affects its physical properties, we 

 may perhaps expect differences between the temperature relations of 

 .wires of different diameters. But there it no obvious reason a priori 

 why differences should exist between specimens of wire of the same 

 thickness from the same sample of platinum. It is clearly desirable to 

 ascertain whether such differences exist, and if they do whether they 

 follow any recognisable law. This is the more important at present 

 owing to the recent proposal made by Prof. Callendar to the British 

 Association to set up standard platinum thermometers made of some 

 one sample of platinum wire. 



Referring to Tables XVII and XVIII, we see evidence of differences 

 between certain of the thermometers. The consistency of the mean 

 values found in different years for pt g in K 3 and K 4 renders it im- 

 possible to ascribe to experimental error the very considerable differ- 

 ences between the values of pt s in these two thermometers, and in the 

 original KI and K2. 



47. The evidence afforded by Tables XVII and XVIII as to differ- 

 ences between K 3 , K 4 , K 5 , and K; is less conclusive. I have thus had 

 recourse to two more sensitive methods of detecting differences. 



The first of these consists in taking the ratios borne to one another 

 by the resistances at the ice, steam, and sulphur points. The taking 

 note of the value of Ri/Ro was in fact recommended originally by 

 Mr. Griffiths as a delicate means of checking the accuracy of individual 



VOL. LXVII. E 



