ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 81 



difference of the thermal e.m.f.s amounting to 2 x 10 7 volt, and it may 

 be difficult entirely to eliminate these. With other forms of brushes, 

 e.g., those made of gauze, the difference was often 1,000 times as great. 

 It was this difficulty which led Lord Eayleigh in 1883 to amalgamate 

 the edge of the disc, and as a further improvement Professor Viriamu 

 Jones and Professor Ayrton used mercury jets instead of brushes. 

 Since in the present apparatus the changes are only 1 in 10,000 of the 

 difference of potential produced in one arrangement of the brushes 

 and less for a second arrangement, it is hoped that mercury contacts 

 will not be necessary. Further experiments will be made in order to 

 obtain greater perfection if such is possible. 



Resistance Standards. — The construction of new mercury stan- 

 dards of resistance in accordance with the specification of the London 

 Conference is being proceeded with, and some of the standards will 

 be completed this year. Similar work is in progress in France, in 

 Germany, in Austria, and in the United States. In the latter country 

 four standards have had all of their constants determined, and the 

 resistance unit so obtained is in very close agreement with that obtained 

 from the old National Physical Laboratory standards. 



In the Committee's Eeport for 1908 it was shown that many 

 manganin resistance coils — some of which were purchased by the 

 Committee in 1895 — were very changeable in resistance, and in conse- 

 quence frequent comparison with mercury standards was necessary. 

 In 1908 it was shown at the Bureau of Standards, and confirmed at 

 the National Physical Laboratory and at the Eeichsanstalt, that these 

 changes were largely due to the effect of moisture on the shellac 

 covering the wire. To eliminate this source of trouble, many of the 

 coils were hermetically sealed in 1909, and it is satisfactory to record 

 that they are now much more constant. The importance of this 

 hermetical sealing is so great when manganin resistances are to be 

 sent to such places as cable stations in the tropics that the attention 

 of instrument manufacturers is drawn to the matter. Standard coils 

 are readily sealed and boxes of coils may be sealed in metal cases. 

 The following figures for standard coils of manganin show the advan- 

 tage of hermetical sealing : — ■ 



1911. 



G 



