to the Wheat stone Bridge. 421 



No. 22 S.W.G., but between these limits an accuracy of 0*1 

 per cent, was easily obtained so far as the sensibility of the 

 method was concerned. 



The accuracy of the standard now remains to be considered. 

 This can be best seen from a description of the method by 

 which the correct length between the points B, was de- 

 termined. 



In order that this length might be of a convenient amount 

 to suit the dimensions of the base-board, it should have been 

 made of copper wire No. 17 S.W.Gr. ; but a piece of this gauge 

 which was bought for the purpose proving unsatisfactory, a 

 piece of No. 16 was taken and stretched till of the required 

 diameter. 



One, Q B, of the two short arms of fig. 1, was soldered on, 

 the other, T C, being left loose. The whole was then screwed to 

 a rough board, and the wire annealed by passing through it 

 a current of about 50 amperes till it was too hot to touch and 

 then allowing it to cool, the operation being repeated ten 

 times. The arrangement represented in fig. 4 was next made. 



Fig. 4. 





C \K±lw> • * 



STfel M 



By following out this figure, it will be seen to be the same 

 arrangement as fig. 3, except that now the place of the plug 

 is taken by the gap P. The standard was a 1-ohm coil of 

 manganin wire, constructed by Messrs. Nalder, Bros. Its 

 terminals rested in mercury-cups from which the various leads 

 were taken, the ends of all wires dipping in the mercury being 

 freshly amalgamated. The leads joining the 10- and 1000- 

 ohm coils together and to the binding-screws T and M respec- 

 tively were short and stout. 



The arm T C was then put approximately in position, and 

 good contact at C was maintained by pressure. To avoid 

 thermoelectric effects this pressure was made with a piece of 

 wood, and in order to see if any such effects were present a 

 commutator, not shown in the figure, was put in the battery- 

 circuit. It was then found that if balance had been obtained 



