420 



THE BRITISH ASSOCIATION 



Table III. 

 Resistance of Coils F and Flat 



IN TERMS OF MeRCURY. 



(Length of Column of i sq. mm. section at o° C.) 



Values found by Lord Rayleigh in the 

 year 1881 : 



Fat i6-o° C 



Flat at 16 -0° C. 



Values found by Mr. Smith in 1908 

 for the resistance of the coils in 

 1881 , assuming them to have altered 

 between 1881 and 1908 by the 

 amounts shown in the British 

 Association Reports : 



Fat i6-o° C. . 

 Flat at 16 -0° C. 



104-805 cm. 

 104-871 cm. 



= 104-808 cm. 

 = 104-874 cm. 



The next table (Table III) gives the values of two of the platinum-silver 

 coils examined by Lord Rayleigh in 1881 and Sir Frank Smith in 1908. It 

 shows that, allowing for the recorded alterations in these coils during that 

 period, the value assumed for the B.A. unit was satisfactorily known. The 

 point of most importance which emerged frorn Sir Frank's measurements 

 of 1908 was the permanence of the two platinum coils. A reference to 

 Table I shows an apparent change of 0-2 in the standard temperature 

 of No. 36 (E) between 1865 and 1867. It would appear from Table IV 

 that this apparent change was not a real one but arose from some error in 

 the 1867 experiments. At any rate Table IV, which is brought up to date 



Table IV. 



Differences between the Values for the Platinum Coils 

 D andE at 16-0° C. 



Note. — A change in temperature of o ■ 1° C. causes a change in resistance of the coils of 

 31 parts in 100,000. 



by the inclusion of Dr. Hartshorn's observations of 1932, gives the values 

 observed for the difference E — D between these coils, and shows that with 

 this one exception this difference has lain between 0-00059 B.A.U. and 

 o - 00063 B.A.U. during the whole sixty-seven years of their life. This will 



