THE MEASUREMENT OF RESISTANCE 



233 



1. Convenience of manipulation; allowing speed to be attained. 



2. No calculation required; that is, the instrument must be 

 direct reading in terms of the accepted standard material. 



3. Freedom from all temperature corrections. 



4. Accuracy to J-fo or Mo per cent. 



One form of such an apparatus is the Hoopes conductivity 

 bridge. This device is an adaptation of the Kelvin double 

 bridge, the scheme being as follows: 



Excess of Weight Scale 



N 



Slide c f Wire 

 i 



i ^ 



Slide f Wire 



Fn;. 128. Hoopes conductivity bridge. 



Hoopes Conductivity Bridge. The standard P and the un- 

 known X are of the same metal; consequently if care be taken 

 that they are at the same temperature, all corrections due to tem- 

 perature are avoided. The arms M, N, m, n are in the same 

 case and made of material of low temperature coefficient, so 

 that their relative values will not change. The sliders c, d, are 



rigidly connected so that when they are moved the ratio vf is 



M 



m 



altered while the relation Ar = is maintained. 



A n 



