CHAP, vi.] ELECTRICITY AND MAGNETISM. 



the solid brass pieces and plugs without encountering 

 any serious resistance ; but when any plug is removed, 

 the current can only pass from the one brass piece to 

 the other by traversing the coil thus thrown into circuit. 

 The series of coils chosen is usually of the following 

 numbers of ohms* resistance i, 2, 2, 5 ; 10, 20, 20, 



50; 100, 200, 200, 500 ; up to 10,000 ohms. 



By pulling out one plug any one of these can be thrown 

 into the circuit, and any desired whole number, up to 

 20,000, can be made up by pulling out more plugs ; thus 

 a resistance of 263 ohms will be made up as 200 -i- 50 



-r 10 + 2 -f I. 



It is usual to construct Wheatstone's bridges with some 

 resistance coils in the arms A and C, as well as with a 

 complete set in the arm B. The advantage of this 



Fig. 133 



arrangement is that by adjusting A and C we determine 

 the proportionality between B and D, and can, in certain 

 cases, measure to fractions of an ohm. Fig. 133 shows 

 a more complete scheme, in which resistances of 10, 100, 

 and 1000 ohms are included in the arms A and C. 

 Y 



