Platinum Thermometry at Kew Observatory. & 



9. Error in the temperature coefficient of the coils, or differences 



between the temperature coefficients of different coils ; 



10. Change of zero or other error in the mercury thermometer inside 



the coil chamber ; 



11. Failure of the box mercury thermometer to give the true tem- 



perature of the coils and bridge wire ; 



12. Insufficient immersion of the thermometer; 



13. Slowness of platinum thermometers in acquiring the true tem- 



perature. 



There are other possible sources of error affecting only certain " fixed 

 point " observations, and mostly not peculiar to platinum thermometry, 

 for instance : 



14. Impurity in the ice used in getting E , or variability in the 



method of treating it ; 



15. Error in the barometer, whether constant or varying, or error in 



the reduction of its readings ; 



16. Impurity in the sulphur used, and other uncertainties in the 



determinations of the boiling point of sulphur ; 



17. Error in the formula assumed for the variation of the boiling 



point of sulphur with pressure. 



I have seen no adequate consideration of these sources of error 

 in any treatment of platinum thermometry, and in the original 

 equipment obtained for Kew Observatory no provision was made for 

 the detection of most of them ; thus our experience may be useful to 

 others. Without at least a 'general idea of their nature, the reader 

 would be unable to judge correctly of the degree of probability attend- 

 ing the conclusions reached, or even of the nature of the evidence on 

 which these conclusions are based. 



8. I propose deferring until the end the discussion of the evidence 

 bearing on the first two items ; only calling attention in the meantime 

 to the fact that without taking a platinum thermometer to pieces it 

 would be very difficult, if not impossible, to distinguish between 

 changes in the resistance of the spiral and changes in the resistance of 

 the thick platinum wires connecting the spiral to the terminals. The 

 (inalterability of a platinum thermometer cannot be proved by experi- 

 ments on an isolated platinum spiral. The connecting platinum wires 

 inside the tube are by no means of negligible resistance. They ought 

 to be very approximately equal in resistance to the compensator loops 

 intended to neutralise their variation with temperature. The resist- 

 ance of the compensator loops has been measured by Dr. Harker with 

 the following approximate results. The data answer to a temperature 

 of about 15 C. 



