424 



THE BRITISH ASSOCIATION 



II parts respectively higher than those given above. However, the above 

 values are corrected to correspond to ' zero current,' and no such correction 

 was made in previous measurements. The magnitude of the correction 

 is shown in Table IV, and it will obviously account for a discrepancy of this 

 order . It was found that when the current through the coil was 0*12 ampere 

 the 1908 values were reproduced to i part in 100,000. It is interesting to 

 note that, owing to the difference in the heating corrections for the two coils, 

 the value for the difference between them varies with the current. The 

 values for this difference, obtained on various occasions, have been given in 

 Table IV, from which we conclude that this difference has remained con- 

 stant since 1865, and that the measuring current used was of the order of 

 o* 12 ampere. (The low value of 1867 has long been considered as due to 

 an observational error.) 



The values obtained for the temperature coefficients are of interest. The 

 following table shows the values obtained on various occasions : 



Temperature Coefficients of the Platinum Coils. 

 B.A. Units per i° C. 



It is evident that the temperature coefficient of the wire is still very near 

 to its original value, although it is far removed from the value for pure 

 platinum (about 400 x 10"^). 



(ii) The Variations in the Alloy Coils. 



Having satisfied ourselves that the platinum coils had remained unchanged, 

 the changes in the other coils were examined. It was not considered neces- 

 sary to take so many observations on these coils as on the platinum ones, 

 but in each case a few observations were made at a temperature slightly 

 above 16° C. and a few at a temperature slightly below 16° C. The value at 

 16 -0° C. was deduced from the two sets. In every case the difference 

 between the two sets of values was consistent with the 1908 value of the 

 temperature coefficient, which is reproduced in Table V, together with the 

 resistance values at 16 -0° C. and the heating correction. 



In the older British Association Reports the values assigned to these coils 

 were usually obtained on the assumption that the mean value of the coils at 

 the temperatures at which they were originally stated to be correct had not 

 altered : in other words, the results were expressed in terms of the mean B.A. 

 unit at the time. Mr. F. E. Smith showed in 1908 that it would almost 

 certainly be more correct to assume that the platinum coils alone had 

 remained unaltered, and our results support this conclusion. Acting on 

 this assumption, and taking the old values for the differences between the 

 coils, Mr. Smith was able to give a table of the values of the coils in terms 

 of the original B.A. unit (1867). Our results enable us to bring this table up 

 to date (Table VI). This table is of great interest as showing the most 



