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XLIV. On a Practical Point in connexion with the Comparison 

 of Resistances. By W. N. Shaw, M.A., Emmanuel Col- 

 lege. Cambridge* . 



[Plate XIII.] 



THE most accurate method at present in use for the com- 

 parison of two resistances differing only by a small 

 fraction of an ohm, is that suggested by Prof. Carey Poster 

 (Journal Soc. Tel. Engineers, 1872). It is a method pecu- 

 liarly suitable for the comparison of standards of resistance 

 and the determination of temperature-coefficients of coils. 

 It gives the difference of two resistances compared, free from 

 errors, which might arise in the ordinary sliding- wire Wheat- 

 stone-bridge arrangement, from uncertainties as to the con- 

 tacts of the ends of the slide-wire, or from the resistance 

 of the connecting-pieces between the ends of the slide- wire 

 and the electrodes of the coils compared. In the process of 

 making a determination by this method, the two coils X and 

 Y have to be made to interchange their positions, with 

 reference to the other resistances of the bridge ; and it there- 

 fore becomes important for the application of the method that 

 the observer should be able to interchange the connexions of 

 the two coils with rapidity and facility. 



This is very efficiently and conveniently provided for in the 

 modified form of the slide-wire bridge devised by Dr. Fleming, 

 and described in the third volume of the Proceedings of the 

 Physical Society (p. 174)|. With the ordinary form of bridge 

 the interchange is more difficult, for the coils to be compared 

 have generally to be kept in water, and the two pairs of 

 binding-screws of the bridge are a considerable distance apart, 

 and thus the shifting of the coils to change their connexions 

 is a somewhat inconvenient matter. 



At the meeting of the Physical Society on Feb. 23, Prof. 

 S. P. Thompson made some suggestions as to the manner in 

 which the ordinary slide-wire bridge might be adapted so 

 that Carey Foster's method could be conveniently applied. 



We have, during the past three years, at the Cavendish 

 Laboratory frequently had occasion to make use of the ordi- 

 nary bridge for comparing coils and determining temperature- 

 coefficients ; and have accordingly found it necessary to devise 

 come simple arrangement by which the easy and rapid inter- 

 change of the resistances under comparison could be provided 

 for. 



* Communicated by the Physical Society. Read March 8, 1884. 

 % Phil. Mag. [5] vol. ix. p. 109. 



