40 ELECTRICAL MEASUREMENTS 



galvanometer. When the circuit is closed the coil will begin to 

 move and as it swings in the field, H, a back e.m.f. will be set up; 

 its magnitude will be 



Consequently the current at any instant will be 





R 



.Let fc'be the damping coefficient when the galvanometer circuit 

 is open. This damping may be due to currents induced in damp- 

 ing loops attached to the movable coil or in a metal frame upon 

 which the coil is wound and, to a slight extent, to the air damping. 

 From the above 



or 



di 

 The term containing -57- is negligible so 



(C 2 \ 

 jg -f JcJ is the damping constant and replaces A; in the 



equation on page 25. 



The case of special importance is when the constants of the 

 circuit are such that the instrument is critically damped. Then 

 if it is assumed that the damping is entirely due to actions in the 

 movable coil itself, 



(30) 

 Introducing T and eliminating r gives 



R/2P 



