Then clearly 



ON STANDARDS OF ELECTRICAL RESISTANCE. 

 "R + r + x __ a 



355 



or 



E + ,-= L' (Z+1000-.r) 







The readings are given in the following Table, 



— .r. 



Zero-point was at 516. 



Eesistance of half length o'f circuit is 21712 millimetres of wire. 



All these values are within necessary errors of observation. The fii'st few 

 values are most to be relied on, as the values of r+R depend nearly directly 

 on 1000— r. 



So many measurements were made in order to find whether the wire 

 tapered towards either end. The similarity of the values found for E, + r 

 shows this better, perhaps, than the dii-ect method before described. 



A set of similar measurements were made with the coil L in the left-hand 

 mercury cups, and equally good results obtained. 



The galvanometer employed was one of Thomson's reflecting galvano- 

 meters, made by Messrs. Elliott Brothers. A short coil was employed. 

 The instrument was placed in a deal box, blackened inside, with large aper- 

 tures to observe through. The spot of light could thus be clearly seen, and 

 the divisions of the scale were sufficiently illuminated to enable the observer 

 to see immediately in which direction the spot of light moved. The instru- 

 ment was sufficiently delicate to show 0-001 per cent, difierence in the ratio 

 of any two nearly equal conductors compared, corresponding to yL miUims. 

 on scale of bridge. 



An ordinary galvanometer was also at hand to find about the place of 

 reading on the scale. 



The balance employed for weighing was by Liebrich of Giessen, and would 

 weigh to ^ijjth of a milligramme with accuracy. The weights were adjusted 

 by Oertling, and again tested by weighing them against the largest weight 

 (50 grms.). Mr. Balfour Stewart was kind enough to test this weight, and 

 foimd its value to be exactly 50-0000 grms. All weighings made in this 

 research were double weighings. 



