14 I >r. ('. ' 'Inn-. / an 



\Vr shi-iiM rather expect the resistance of the bridge wire to im 

 especially near its centre, through constant rubbing by the coi; 

 pici-f, .-'inl the figures obtained in Calibrations III and IV somewhat 

 favour thi> view. Too much significance ought not, however, to !>< 

 KM! to the small differences apparent. 



Some of the differences in the coil values in Calibrations II, III, and 

 IV may be experimental errors ; but the changes shown in the case of, 

 at least, E and G are, I think, too large to be accounted for in this way. 



8 12. Perhaps the clearest evidence of the reality of coil variations is 

 that afforded by an examination of the thermometric results obtained 

 with the different coil combinations. It would occupy too much space 

 to go into details, so I merely record in Table V the mean differences 

 between the values of RO, RI, and R x (resistance in sulphur vapour) 

 obtained with the two coil combinations used during different specified 

 epochs. The unit in the table is 0*01 mm. of bridge wire, answering 

 approximately to CT'OOl C. with the ordinary thermometers. 



In the final means equal weight is allowed to each observation, so 

 that some thermometers exert more influence than others. 



During some of the epochs especially 1895 and the first part of 

 1899 the data with any one thermometer were very scanty. Again, 

 it must be remembered that the part of the bridge wire at which readings 

 are taken differs according to the thermometer used, and also varies for 

 any one thermometer according to the temperature of the coils, the 

 position of the bridge centre, and the difference between the leads. It 

 is also different in the ice, steam, and sulphur point observations, 

 fluctuating considerably in the latter two wises with the barometric 

 pressure. Thus the fluctuations in the table amongst the results for a 

 common epoch are due to many causes. 



$ 13. It is, I think, most instructive to start with Calibration III 

 nvide in July, 1897. During the rest of that year the results from the 

 two coil combinations show almost perfect agreement; in 1898 the 

 results drift apart, and the drift is accentuated in 1899 prior to Cali- 

 bration IV. Again, for some months after that calibration there is an 

 excellent agreement, though a tendency to drift soon manifests itself. ' 



Calibration II seems less successful, but it was made by an observer 

 different from the one who took the readings on which the results in 

 Table V are based. A different standard of plug tightness does not 

 influence all plugs alike, and in my experience the personal equation in 

 this matter requires to be reckoned with. 



The data from March 12 to 19, 1897, were so outstanding that they 

 are given separately. The exposed parts of the box had, I believe, 

 Ixjen cleaned shortly before, and conceivably one of the coil supports 

 ni iv have got a knock. There was, however, no suspicion of this at 

 the time. This fact emphasises the necessity of a constant outlook for 

 possible changes. 



