Platinum Thernwmetry at Kew Observatory. 23 



as a corresponding experiment with the same thermometer in ice. This 

 is necessary on account of possible changes in the resistance of the 

 dry cell. 



At the sulphur point the heating effect is only about one-third of 

 what it would be if it depended only on the value of i-p in the spiral, 

 and even at the steam point the heating is less than three-quarters of 

 what it would be on this erroneous hypothesis. 



21. If the heating effect due to a given i 2 p were the same at all 

 temperatures, Tables VII and VIII would justify the conclusion that 

 with 100 ohms in the battery circuit all that is to be feared is a trifling 

 fall, of the order 0- 003, in the apparent zero E as the cell deteriorates. 

 But, according to Tables VIII and IX, the true conclusion is that even 

 with 100 ohms in circuit the heating effect is likely to diminish the 

 fundamental interval by several thousandths, and the ice-sulphur 

 interval by not much less than a hundredth of a degree. The use of 

 20 instead of 100 ohms in sulphur point observations would largely 

 increase the uncertainty. 



The changes in R g - EQ and RI - RO, due to varying heating effect, 

 are in the same direction, so that the effect on pt s is relatively small, 

 and, in view of the numerous uncertainties existent, I have not thought 

 it worth while to attempt a " reduction to an infinitely small current." 

 It is clear, however, that unless the current can be materially reduced 

 by the employment of a more sensitive galvanometer, the necessity of 

 providing for this reduction must be kept in view in all work of the 

 highest accuracy. 



Before quitting the subject, I would remark that the diminution of 

 the heating effect of a given i 2 p as the temperature rises is only what 

 we should expect from the known increase with temperature of both 

 specific heat and radiating power. It is possible that heating effect 

 experiments with thermometers of other metals than platinum, or with 

 spirals of varying gauge of wire, might prove a useful method for 

 investigating radiating power. 



Error in the Temperature Coefficient of the Coils or Differences between the 

 Temperature Coefficients of Different Coils. 



22. The standard temperature selected by Mr. Griffiths was 20 C., 

 and he" found for the relation between the resistance r T of a sample of 

 the coil wire at temperature r, and its resistance r 20 at the standard 

 temperature, 



TV = r 20 [1+0-00026 (T- 20)] (2). 



All temperature corrections have been deduced by (2) since the box 

 came to Kew Observatory. 



Suppose after the calibration corrections are applied, that the 

 observed box reading makes p the resistance of a certain platinum 



