174 



ELECTRICAL MEASUREMENTS 



an inaccuracy in placing the zero of the scale opposite the end 

 of the slide wire. The resistances between e' and the battery 

 terminal h, and between / and the battery terminal i act as 

 prolongations of the corresponding ends of the slide wire. Let 

 rii denote the number of millimeters of the wire which would 

 have the same resistance as e'h, and n 2 the corresponding 

 number for fi. Then if HI and n 2 were known, their effect 

 would be allowed for by writing 





1,000 - 



The values of HI and n z may be determined as follows : 

 After thoroughly cleaning the surfaces of contact insert the 

 straps at a and b. Place at c a coil of A ohms and at d a coil of 

 B ohms. A and B must be different. Call the reading of the 

 slider when the balance has been obtained li. Then 



B 1,000 - h + n 2 



Interchange A and B and obtain a new balance at Z 2 . 

 Then 



g 1 2 + m 



A == 1,000 - Z 2 + n 2 



Similarly 



A + B 1,000 



D 7 



> t2 



A +5 1,000 

 Bh- 



A-B 



B(l,000 - k) - A (1,000 - ZQ 

 A -B 



Extension Coils. If, in measuring X, the balance point fal 

 near one end of the bridge, the shorter section of the wire cann< 

 be determined with accuracy. Again, if equal coils are beii 

 compared, greater accuracy in setting s may be desired, 

 either case, it would be advantageous to use a longer wire, 

 this would be inconvenient, the same result is attained by insei 



