THE MEASUREMENT OF CURRENT 43 



and 



112,000 



I 



. (35o) 



The value^pf A is generally between 600 and 800, depending 

 mainly on the" breadth of the coil. The quantity n depends to a 

 certain extent on the size of the wire and will probably lie between 

 1.4 and 1.6. 



As the resistance of the circuit is increased, the practical 

 application of the results of this discussion become more and more 

 difficult, especially if a quick-working (short period) instrument 

 is desired. 



Auxiliary Damping. If the galvanometer is to be used in 

 circuits of varied resistances, the damping will vary to corre- 

 spond, rarely being such that the instrument may be read quickly; 

 in such cases recourse is frequently had to auxiliary magnetic 

 dampers which are in effect closed loops of wire attached to the 

 coil and swinging in the same field. A familiar example of this is 

 the damping device used on direct-current ammeters and volt- 

 meters, the coil being wound on an aluminum frame or bobbin. 



Possible Adjustments. An instrument not specifically de- 

 signed for a given piece of work may sometimes be made more 

 effective by special adjustment. The things which may be 

 varied are: 



1. The total resistance of the circuit. This may be adjusted 

 by resistances in series or in shunt with the galvanometer, as 

 necessary. 



2. The field strength. This may be decreased by using a mag- 

 netic shunt or increased by using a second set of magnets in 

 parallel with the original one. 



3. The torsion constant r. The suspension may be changed. 



4. The damping, by the use of an auxiliary damping loop. 



5. The moment of inertia P. This may be increased by 

 placing weights on the movable elemen-t. 



