Platinum Tkcrmometry at Kew Observatory. 57 



4. If possible, the causes of wanderings in the bridge centre and of 



thermo-electric currents not eliminated by the Griffiths key 

 should be removed. 



5. Unless the plug resistance uncertainties can be reduced, it would 



seem desirable to increase the resistance inside platinum 

 thermometers intended for very exact measurement of very 

 slowly altering temperatures, such as occur in "fixed point" 

 observations. 



6. Unless the heating effect of the current can be exactly ascertained 



and allowed for, the sensitiveness of the galvanometer should 

 be largely increased, so as to render unnecessary currents whose 

 heating effects will sensibly influence the readings. 



7. It is certainly desirable that the entire room containing the appa- 



ratus should be kept approximately at the temperature accepted 

 as the standard temperature for the coils. At the same time 

 it is not desirable that the observer should be continually 

 exposed to so high a temperature as 20 C. A temperature 

 such as 62 F. is very much healthier, at least in winter, and is 

 more bracing. There is no obvious reason why there might 

 not be one standard temperature for winter and another for 

 summer, so long as the temperature relations of the resistance 

 coils are accurately known. 



Further Experiments Wanted. 



54. I should like to see experiments made on the following points : 



1. The true law of the variation of the boiling point of sulphur with 



pressure ; 



2. The behaviour of new platinum thermometers under a variety of 



conditions ; 



3. The effect of long or frequent exposures to temperatures extending 



from that of sulphur vapour to the highest temperature for 

 which it is claimed that platinum thermometers are suitable ; 



4. The effect of exposure to low temperatures, such as are now attain- 



able by means of liquid air ; 



5. The degree of accordance in the results deduced by Callendar's 



formula from observations taken with different platinum 

 thermometers varying in their 8 at two or three fixed 

 temperatures differing considerably from that of sulphur 

 vapour, these temperatures being obtainable with a degree 

 of accuracy not inferior to that attained in sulphur point 

 observations ; 



6. The cause of the curious phenomenon described in 48 and 49. 



55. The great majority of the observations on which this paper 



