Platinum Therinometry at Kew Observatory. 13 



of mercury thermometers, and it is only fair to recognise that they do 

 not in this respect necessarily suffer by comparison with platinum 

 thermometers. 



Coil Changes, 



11. The box was originally calibrated at Cambridge by Mr. E. H. 

 Griffiths, who referred everything to a mean box unit based on the 

 eight coils H to A. Since the box came to Kew Observatory it has 

 been thrice calibrated, with the aid of a convenient apparatus designed 

 "by Dr. Harker. The observations were made on the first of these 

 occasions by Dr. Harker ; on the other two occasions mainly by 

 Mr. W. Hugo, Senior Assistant at the Observatory. I took a certain 

 number of check observations during the two last calibrations, and 

 made all the calculations necessary to construct the correction tables 

 based on these and on Dr. Barker's calibration. In doing so, I 

 followed Mr. Griffiths' procedure, except in some minor details. 



The results of the four calibrations were as follows : 



Table IV. Coil Values in Terms of Mean Box Unit. 



The considerable differences between the results of Calibrations I and 

 II are probably mainly due to changes attending the reorganisation of 

 the box in the spring of 1897. Coil I broke at one end, and had to be 

 resoldered just before the last calibration. 



It is improbable that the algebraic sum of the resistances of the eight 

 coils H to A was unaffected by the box changes in 1897, and thus the 

 mean box units in Calibrations I and II were almost certainly slightly 

 different. The data obtained with the thermometers, as will be seen 

 later, suggest a slight increase at this time in the mean box unit ; and 

 this is, at least, consistent with the apparent change in the bridge- wire 

 resistance. 



