DOUBLE BRIDGE. 



377 



coils of 1000 ohms; the second of 100 coils of 20 ohms. The 

 sliders rest on knobs which connect the successive coils, and are 

 moved by handles. The two contacts of the slider A^ have 

 between them coils of 1000 ohms, and therefore a resistance 

 equal to 100 coils in the other dial. 



The handle of the second dial being on the first contact, the 

 handle of the first is moved until the resistance introduced is 

 sufficient, to within a coil, to establish equilibrium. Moving 

 then the handle of the second dial, we seek, to within a coil of 





Cadr 



p 0000000 



Fig. 199. 



the second series, the contact which gives the closest approximation 

 to equilibrium. 



If the contact A l stops at the Nth coil, and the contact C 

 on the tfth, we shall have 



a loooN, 



--2 IOOO, 



and therefore 



b' (101 - N - 2) iooo + (ioo - n) 10 10000 



ioooN + ion 



looN 



I . 



When the ratio sought is near unity, N is nearly 50 ; an 

 error of one unit in the value of n only produces an error of 



in the ratio of the resistances. The approximation is less 



50000 



