Platinum Thermomctry at Kcw Observatory. 



53 



The numbers enclosed by brackets were used in my original calcula- 

 tion, so their coincidence with the observed values is without signifi- 

 cance. The coincidences could have been improved by supposing that 

 the amounts of pure platinum in KS, K 4 , KS, and K^ are only nearly 

 and not absolutely the same, and by slightly altering the values 16/10 

 and 36/10 assumed for the ratios borne by the resistances of the foreign 

 material at the steam and sulphur points to the resistance at the ice point. 



That the above coincidences are purely accidental seems improbable, 

 but what the real nature of the physical fact underlying the figures is, I 

 do not know. 



Accuracy of the Observations. 



50. It is difficult to say what is an equitable basis on which to dis- 

 cuss the question. Presumably if the room containing all the appa- 

 ratus were kept at a constant temperature, and a uniform source of 

 current were employed ; and if arrangements were made for taking the 

 bridge centre, for reversing the battery, and for interchanging the leads 

 and proportional arms at every observation, a variety of sources of 

 uncertainty affecting the Kew results would be much reduced. To 

 provide, however, for all possible interchanges must add to the cost of 

 platinum thermometry, and tend to make observations more tedious. 

 I am thus disposed to think that, except in the case of the most refined 

 physical work, where apparatus is abundant and time and expense of 

 relatively small account, the conditions are unlikely to be more favour- 

 able, and will in general be less favourable, than those prevailing during 

 the experiments at Kew Observatory. 



Under any circumstances a great deal may depend on whether it is, 

 or is not, deemed essential to make a fixed point observation either 

 in ice or steam immediately before or after each series of observa- 

 tions. If a fixed point observation be not taken, we may presumably 

 expect uncertainties in the calculated temperatures of the same order 

 as the fluctuations observed at Kew in E or RI ; while if a fixed 

 point observation is taken, we may presumably expect, when mea- 

 suring a steady temperature between and 100 C., a similar degree 

 of accuracy to that met with in the determinations of fundamental 

 intervals in the present inquiry. 



