70 ELECTRICAL MEASUREMENTS 



Suppose the core to remain fixed in position. If it is but weakly 

 magnetized by the current, the strength of the induced polo will 

 be roughly proportional to the current. This pole reacts with 

 the field of the solenoid which is proportional to the current; so 

 the attraction is approximately proportional to the square of the 

 current. If the magnetizing field is so strong that the core is 

 "saturated/ 7 any further increase of the current alters the 

 strength of the induced pole but little and the attraction is 

 approximately proportional to the current. 



Thus, as the current is gradually increased from zero to a high 

 value, the law of the pull changes from that of the square to 

 that of the first power of the current, and the pull is also modified 

 by the change in the position of the iron core due to the yielding 



FIG. 34. Illustrating principle of magnetic-vane instruments. 



of the spring or gravity control. The net result is that the scales 

 of direct current soft-iron instruments are not uniformly divided. 



The quality of the iron is important. In order that the indi- 

 cations be independent of the previous magnetic history of the 

 iron, it should be as free from hysteresis as possible. For direct- 

 current work soft-iron instruments are now employed where 

 cheapness and robustness of construction are essential and a 

 moderate accuracy will suffice. 



In alternating-current work this construction is employed in 

 ammeters where its use avoids the necessity of taking large 

 currents into the movable parts of the instrument. 



Magnetic-vane Instruments. The principle involved in the 

 magnetic-vane instruments is sufficiently illustrated by Fig. 34. 



As shown in the figure, E is a soft-iron vane fixed to the spindle 



