Platinum Tliermometry at Kew Observatory. 31 



thermometer. This last information is of most consequence in the case 

 of the experiments in ice, because the errors must then increase as the 

 temperature of the room rises, and during the immersion experiments 

 the room temperature was much below its ordinary summer value. 



Some of the figures are based on two or three complete independent 

 experiments, but others depend on only one experiment. In most 

 cases there were in addition several check experiments, with only two 

 or three different immersions, but I have left these out of consideration 

 in constructing the table. The results cannot claim any very high 

 degree of precision, because it is difficult to secure uniformity in the 

 conditions, more especially when the immersion is in ice. The difficulty 

 is greater the less the immersion. When the " bulb " only is immersed 

 in ice, the reading after attaining a minimum usually rises somewhat. 

 The phenomenon seems mainly due to melting of the surface ice, with 

 consequent deterioration of contact between it and the thermometer ; 

 but possibly a difference in the circumstances under which the thick 

 platinum leads and the compensator wires cool may possess some 

 influence. In taking the minimum readings, as I have normally done, 

 I have of course, if anything, favoured the thermometers. This has 

 also been done to some small extent in assuming that "bulb " + 10 cm. 

 is in all cases ample immersion. As a matter of fact, an unmistakable 

 difference was observed in the reading of several of the thermometers 

 when the immersion was increased from " bulb " + 10 cm. to " bulb " 

 + 12 '5 cm. This difference was not, however, always in one direction, 

 .the phenomena in one case distinctly suggesting that cooling an 

 additional 2-5 cm. of the compensator wires had more effect than cool- 

 ing an additional 2 '5 cm. of the platinum leads. 



The experiments are not sufficiently varied to justify any too con- 

 fident explanation of the causes of the large differences between the 

 different thermometers. Thus the pre-eminence of the errors in K 4 

 might at first sight be attributed to the fact that the tube is of porcelain 

 and of exceptionally large diameter ; but KI is also of porcelain, while 

 KG has a much larger diameter than K;. Perhaps the most probable 

 explanation is that K 4 is short for its tube, so that the mica frame 

 which is exceptionally short only conies to within about 1 cm. of the 

 end of the tube. 



Ceteris paribus, we should expect that with a given immersion, the 

 error would vary approximately as the difference between the tempera- 

 ture of the immersed portion of the tube and that of the air of the 

 room, but to confirm or refute this conclusion would require a consider- 

 able number of careful experiments in which either the temperature of 

 the bath or that of the room alone is varied, all the other conditions 

 remaining unchanged. We cannot make use of the experiments in ice 

 and steam for this purpose, because the conditions under which the 

 transfer of heat occurs are radically different. We can meantime only 



