C 507 ] 



LVII. On a Modified Resistance-Balance. 

 By Professor Silvanus P." Thompson, B.A., D.Sc* 



[Plate XVI.J 



SEVERAL forms of Resistance-Balance modified for the 

 purpose of facilitating the accurate measurement of small 

 resistances or small differences of resistance are known. The 

 modification due to Prof. Fleeming Jenkinf, and the Double- 

 bridge of Sir W. Thomson J are perhaps the earliest and 

 least-known of these. KirchhofFs arrangement, as further 

 modified by Matthiessen, is well known under the name 

 of the u divided-metre " bridge. The method of using this 

 form of balance suggested by Professor G. Carey Foster §, in 

 which the difference between two nearly equal resistances is 

 expressed directly in terms of a length of a graduated wire 

 whose resistance is very accurately known, is so extremely 

 reliable, that probably no one who once adopts this method of 

 testing resistances will ever go back to the cruder and less 

 accurate methods. The essence of the method lies in arranging 

 the coil whose resistance is to be measured and the standard 

 coil to which it approximates in series with the graduated 

 wire, in such a manner that the positions of the two coils can 

 be interchanged. The points of the wire at which the poten- 

 tial is the mean between the potentials at the two ends of 

 the series having been determined, both before and after the 

 interchange, the resistance of the length between these two 

 points is the desired difference. A special modification of 

 the balance for facilitating Foster's method was described to 

 the Physical Society in Dec. 1879 by Dr. J. A. Fleming ||; 

 and has been since then systematically used in the Cavendish 

 Laboratory by the Committee on Electrical Standards for 

 the comparison of standard coils. Although extremely con- 

 venient, Dr. Fleming's Balance appears to me to be open 

 to several objections. In the first place, the interchange of 

 the two coils, though much more easily accomplished than in 

 the older divided-metre form, is effected by lifting the termi- 

 nals of each coil out of one of two pairs of mercury-cups, and 

 removing them to the other pair. This operation is not 



* Communicated by the Physical Society. Bead February 23, 1884. 

 t Report of Committee on Electrical Standards. 

 % Proc. Roy. Soc. vol. xi. p. 313 (1861). 

 § Proc. Soc. Telegr. Engin. vol. i. pt. 2, p. 196 (1872). 

 || Proc. Phys. Soc. vol. iii. p. 174 ; Phil. Mag. ser. 4, vol. ix. p. 109 

 (1880) ; Telegr. Journal, vol. viii. p. 3 (1880). 



