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XIII. A Direct-reading Conductivity -Bridge. 

 By Rollo Appleyakd*. 



WHEN it is required to check the conductivity of several 

 tons of copper wire, of the same nominal diameter, 

 the method employed generally resolves itself into making a 

 careful test of the conductivity of the wire of a selected hank T 

 and afterwards balancing a length of this on a slide-bridge 

 against successive equal lengths of the remaining hanks of 

 wire of that diameter. If an ordinary slide-bridge is em- 

 ployed, the scale-readings corresponding to the balance- 

 position of the sliding contact are not directly proportional 

 to the successive conductivities. To arrive at the conduc- 

 tivity a certain amount of arithmetic is necessary. If, 

 however, the slide-bridge is arranged somewhat differently, 

 the readings of the slider can be made to be directly pro- 

 portional to conductivities. 



Let a be the resistance of the selected wire of known con- 

 ductivity k a , and let b be the resistance of any of the remaining 

 wires w hose conductivity kf> is to be determined. The third 

 arm of the bridge is a fixed resistance d„ and the fourth arm 

 is the slide-wire c, the resistance of which can be adjusted to 

 balance d by moving the slider. 



We have therefore, 



* Communicated by the Physical Society : read 8th March, 1901. 



