Electrical Resistances with a Constant Current, 317 



comparison it is clearly necessary that currents of known or 

 constant strength should be used in their measurement. 

 When the Wheatstone's bridge is employed in the usual 

 manner, the current passing through the unknown resistance 

 will, of course, vary with the magnitude of this resistance, 

 being smaller when it is high than when it is low ; but by a 

 very simple modification of the common arrangement, which 

 I have used extensively during the last year, it is easy to 

 ensure having currents of uniform strength throughout a 

 series of measurements. 



In the figure, x, nx, a, na are the four arms of a Wheat- 

 stone's bridge, S is the unknown resistance, and R is a box of 



resistance coils which is inserted in the same arm. If E 

 denote the electromotive force of the battery, B its internal 

 resistance, and C the current which passes through the arm 

 containing S, then, when there is a balance, 



71 + 1 B rc(a+aQ a 



a + x + n{a + x) 



nE 



(n + l)B + n(a + x)' 



From this expression we can find what value x must have in 

 order that the current through the unknown resistance may 

 be of any definite strength. Having determined this value, 

 we insert resistance equal to n times its amount in the arm 

 nx y and adjust the resistance in the box R until a balance is 

 obtained. We then know that the resistance of R + S is equal 

 to x; that the resistance required to be measured, S, is equal to 

 that of the arm x less the resistance employed in R; and that 

 a current of the desired strength, C, is passing through it. 

 Phil. Mag. S. 5. Yol. 15. No. 95. May 1883. 2 A 



