Platinum Thermometry at Kew Observatory. 29 



of mercury of about 10-degree divisions exposed to the influence 

 of the surrounding atmosphere, whose temperature is, say, 20 C. 

 This emergent column possesses a mean temperature somewhere 

 between and 20, and so occupies a larger volume than if all were 

 at ; on this account alone the thermometer must read too high. The 

 emergent column has a second influence tending in the same direction ; 

 it serves, conjointly with the glass tube, as a path for the conduction of 

 heat to the bulb. In high-temperature measurements the error due to 

 a long emergent column may amount to several degrees, the thermo- 

 meter in this case reading too low, The conduction along the stem is 

 usually much the less important influence of the two. 



A platinum thermometer is not wholly free from immersion diffi- 

 culties. It is not sufficient to have the spiral inside the bath or chamber 

 whose temperature is desired, so that the defect in a platinum thermo- 

 meter is most analogous to the second source of error described in the 

 mercury thermometer. 



If, for instance, a platinum thermometer is buried in ice to only a 

 little over the top of the spiral, the tube emerging in a room at 20 C., 

 the heat communicated to the spiral through the surrounding air or 

 through the connecting wires raises the reading appreciably. Similar 

 insufficient immersion in the hypsometer sensibly lowers the reading. 



It was difficult to settle on a suitable basis- for comparing the effects 

 of insufficient immersion in the different platinum thermometers. The 

 spiral is wound on a mica frame, and usually one of the small mica discs 

 which hold the wires apart is situated immediately above the top of the 

 frame. When this occurred I have taken the " bulb " of the thermo- 

 meter as extending to the first mica disc ; but in one case, where the 

 interval between the first disc and the frame was considerable, I took 

 instead the top of the frame itself. As a rule, the spiral stops short of 

 the top of the mica frame by about f cm. ; so the definition is at least 

 a lenient one to platinum thermometers. 



The tubes of K 5 , K 6 , and K 7 being of transparent glass, the length 

 of the " bulb " was visible from outside ; but in KI and K 4 the tubes 

 had to be taken off to allow of the requisite measurements being made. 



In the immersion experiments in steam the thermometers were 

 carried in the usual way by a cork, about 3'25 cm. long, fitting the 

 neck of the hypsometer, and the immersion was counted up to the 

 lower face of the cork. Thus fully 3 cm. of the tube immediately 

 above the portion exposed to the steam was protected from direct 

 cooling by the air. 



28. Table X gives particulars as to the results obtained. It 

 includes the errors observed with the various immersions, measured on 

 the hypothesis that an immersion of " bulb " + 10 cm. was in all cases 

 sufficient. It also gives the lengths of the " bulbs," and the mean tem- 

 perature in the room during each series of experiments with each 



