THE BALLISTIC GALVANOMETER 



105 



is rotated through 180 by a spring, thus cutting the lines due to 

 the fixed coils. 



This is a secondary instrument. The number of lines cut 

 by the movable coils is 



n = KI 



where / is the current in the fixed coils and K an experimentally 

 determined constant the value of which is furnished by the instru- 

 ment maker. It is seen that the number of lines cut may be 

 varied by altering the current through the fixed coils. 



FIG. 53a. Duddell inductor. 



When a mutual inductance is employed, the galvanometer is 

 placed in series with the secondary winding. The primary cir- 

 cuit may be arranged so that a measured value of the current 

 may be suddenly reversed. In this case 



CD 



where m is the mutual inductance, I the primary current and r 

 the resistance of the secondary or galvanometer circuit. 



A common form of mutual inductance used for this purpose 

 consists of a long, straight primary coil of one layer wound on a 

 non-magnetic core, and a short secondary coil wound outside the 



