PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 



47 



mental interval, with a view to ascertain the change on annealing, which 

 is then carried out by heating two or three times to about 1000° for 

 several hours, with slow cooling, the thermometers with glass tubes being 

 temporarily placed in porcelain ones for this purpose. The fundamental 

 interval is then taken again, and if this is not considered sufficiently near 

 the desired value, it can be lowered by cutting out the requii'ed amount 

 from the looped end of the wire and re-fusing, or raised by stretching 

 judiciously with platinum-tipped pliers the lowest few inches of the wire, 

 which is unwound for the purpose. Care must be taken after each re- 

 adjustment to remove any possible new strains introduced by a thorough 

 re-anneal before measurement. In the absence of definite evidence in its 

 favour, it was not deemed desirable for this first set of thermometers to 

 heat the wire for some hours electrically to 1400° or 1500° C, as is usual 

 in careful work with wires of platinum and the allied metals employed for 

 thermo-j unctions. 



After the final adjustment of the FI and final anneal, systematic obser- 

 vations of the zero, steam, and sulphur points of the four thermometers 

 were made from time to time with the resistance-box described in the 

 text. A new calibration of the box-coils and bridge wire was made in 



February 1 903, and the values of the relation ' and of the 8 found 



since that date are tabulated for each thermometer. From this summary 

 it will be seen that there appears to be a small but systematic difference 

 between thermometers 1 and 3 on the one hand, and 2 and 4 on the oclier, 



=-L and of c. 



Kg 



this being noticeable both on the values of 



The values of ?J vary from 1-38709 in BAj to 



R 



1-38881 in BA, 



the mean of the four being 1-38786, which is a little higher than the 

 mean value found by Chree for the group of seven thermometers studied 

 by him, namely, 1-38702. 



The mean values of the c are : 



The mean ^ of the six thermometers observed in sulphur in Chree's 

 experiments was 1-503, the maximum being 1-509 and the minimum 

 1-498. The mean values of the Rp, R,, and FI for the period from 

 February 12 to August 31 are also given. In view of the uncertainties 

 in the measurement of the temperature of the box-coils, which are of 

 platinum silver not immersed in a liquid, and also of small irregularities 

 in the behaviour of the plug-contacts, the experiments afford no certain 

 evidence of systematic change in any of the thermometers, unless it 

 be a small rise in the fundamental coefficient and correspondino- fall in 

 the 8 of BAj. ° 



