MEASUREMENT OF CURRENTS BY FALL OF POTENTIAL. 251 



A plug placed in a allows the entire current to pass through the 

 galvanometer G ; placed at b, it introduces both the shunt s and the 

 compensating resistance /> ; and so on for the other positions c and d. 

 When the plug is at e, it closes the galvanometer, thus making it 

 safe. 



A system of shunts, provided with compensating resistances, 

 forms the necessary complement of any accurate galvanometer 



o-o- <r 



(Fig. 163). The shunts have usually the resistances -, , , 



9 99 999 

 so that their multiplying powers are then 10, 100, 1000. 



The use of shunts greatly facilitates galvanometric operations, 

 but it involves some inconveniences which require great care 

 whenever exact measures are to be made. One of the principal 

 arises from changes of temperature, which, as they affect unequally 

 the wire of the coil and that of the shunt, alter the value of the 

 multiplier to an unknown extent. 



868. MEASUREMENT OF CURRENTS BY FALL OF POTENTIAL. 

 An arrangement frequently adopted, and which is particularly ad- 

 vantageous in the case of powerful currents, consists in placing 

 the ends of the galvanometer wire in connection with the two 

 points A and B of the principal circuit, comprising between them 

 a resistance s which plays the part of a shunt. If the resistance 

 of the galvanometer is considerable in comparison with that of the 

 shunt, the compound resistance 



= = 



1 



differs little from the resistance s, and the intensity I 1 from in- 

 tensity I, so that the introduction of the galvanometer as a shunt 

 does not sensibly alter the intensity of the total current. We can 

 then estimate directly the difference of potential E = V l - V 2 between 

 the two points A and B by comparing the deflection 8 obtained with 

 the deflection 8 given by a standard electromotive force E , such as 

 that of a Daniell's cell, the internal resistance of which is very small 

 compared with that of the galvanometer. We have 



(36) = s, from which ,.* 



6 6 Q S S S Q 



