Platinum Thermometry at Kew Observatory. 



21 



points, with a given value of K', and the relatively large effect of an 

 addition of 5 ohms to the battery circuit when the rest of the resistance 

 is only 20 ohms. 



No very serious error would arise from treating the heat supplied as 

 independent of the temperature, at least up to 445 C., so long as there 

 is a large resistance of the order of 100 ohms in the battery circuit ; 

 and under these conditions a moderate change in the resistance of the 

 battery itself is not of much consequence. 



19. At any one fixed point, as already stated, we may reasonably 

 expect the rise of temperature of the spiral to be proportional to the 

 supply of heat. . It is desirable, however, to check this conclusion by 

 experiment. I thus compare in Table VIII the above theoretical 

 results with some experiments made with thermometers Ko and KS at 

 the steam point in June, 1897. In these the additional resistance in the 

 battery circuit beyond that of the battery itself was given several 

 values intermediate between 20 and 100 ohms. 



Assuming 



Heating effect/heat supplied = C, a constant, 



I determined C by equating the observed and calculated values of the 

 difference of the heating effects with the two extreme additional resist- 

 ances 20 and 100 ohms. This quantity is denoted by H 2 o - HIOO, and 

 a similar notation is employed for the other similar difference effects. 

 The differences in the table are really lengths of bridge wire, but for 

 practical purposes they may be treated as temperatures. 



Table VIII. Heating Effects of Battery Current at Steam Point. 



The resistance of the battery itself was not determined on either 

 occasion ; but from the large size of the heating effects compared to 

 those of later experiments with an older cell, I am inclined to suppose 

 that the resistance was nearer than 5 ohms. The values calculated 

 with R' = 5, however, unquestionably present the closest accordance 

 with experiment. In fact, taking into account the difficulty of the 

 experiments, the agreement in this case is altogether too good, and 

 must be largely a matter of luck. However this may be, there is, I 

 think, every reason to suppose that the values deduced for the heating 



