334 COMPARISON OF RESISTANCES. 



and consequently 



^ E v 

 <$,= - - -dr. 



The deflection A x produced by a single coil is 



kEr 



''To^V 



it follows that 



dr p + 2r 8 l 

 r p + r A/ 



940. The differential galvanometer also enables us to compare 

 very different resistances, provided we shunt the coil corresponding 

 to the smallest resistance /. Let m' be the multiplying power of 

 the shunt, / and *' the intensities ; the needle being at zero, we 

 have 



consequently 



and if the galvanometer is adjusted 



When the shunt is insufficient, the two coils are put in separate 

 circuits, one with the resistance r and n couples, the other with the 

 resistance r'> a single couple and the shunt. This method is also 

 often used in measuring the resistance of an insulator ; we have, in 

 that case, 



r = nm' r' . 



941. We shall give further the method of Sir C. W. Siemens* 

 and that of Fleeming Jenkin,f as allied to the use of the differential 

 galvanometer. 



* C. W. SIEMENS. British Association Report. 1867. Reprint, p. 142. 

 t JENKIN. British Association Report. 1867. Reprint, p. 144. 



