52 



ELECTRICAL MEASUREMENTS 



come to rest are reduced. A familiar example of the use of 

 shunts to extend the range of galvanometers is found in direct 

 current, moving coil ammeters. 



Where it is desired to compute the total current in a circuit 

 from the indication of a shunted galvanometer, an exact knowl- 

 edge of the resistance of both shunt and galvanometer at the 

 time of use is necessary. Attention must be given to possible 

 sources of error, such as defective contacts and changes of resist- 

 ance due to temperature. 

 Let / = line current. 



I = galvanometer current. 



I s = shunt current. 



RG resistance of galvanometer. 



S = resistance of shunt. 

 Then 



\~~S / is called tne multiplying power of the shunt. The 

 ordinary arrangement of a shunt box for use 



, i ~ T 



with a reflecting galvanometer is shown in 

 Fig. 20. By changing the position of the 

 plug, definite portions, usually J^ 0| i^ 00 , O r 



J5I 



FIG. 20. Diagram for ordinary shunt box. 



be sent 



FIG. 21. Dia- 

 gram for universal 

 shunt box. 



the galvanom- 



The Ayrton-Mather Universal Shunt -The universal shunt 



^ siown diagrammatically in Fig. 21. A high resistance, 



r ohms, is permanently connected across the galvanometer 



unals, one of which, T r , is permanently connected to the 



-mal circuit. By the proper arrangements, the other lead 



