THE MEASUREMENT OF CURRENT 



71 



which carries the pointer and the inner end of the controlling 

 spring. 



On the passage of the current through the coil CD, in which 

 the above arrangement is inserted, the iron is magnetized, the 

 like poles repel each other and the needle is moved over the scale. 

 An air damper is usually added. 



There are many variations on this fundamental design, which 

 is used for both ammeters and voltmet 



Weston Soft-iron Instruments. In the Weston soft-iron 

 instruments the arrangement is as indicated by Fig. 35. 



A thin piece of soft iron 

 abc of the form shown is 

 bent to conform to a cylin- 

 der; de is another thin piece 

 of iron of rectangular form 

 so bent that it is coaxial with 

 abc. It is rigidly attached to 



FIG. 35. Weston soft-iron instrument. 



the spindle. The coil has a large opening at the center in which 

 this arrangement is placed with the spindle coinciding with the 

 axis. 



On the passage of the current the neighboring edges of the 

 iron are similarly magnetized, the like poles repel each other and 

 the index is forced over the scale. This construction is employed 

 in both ammeters and voltmeters intended for use with alternat- 

 ing currents. 



The General Electric Co.'s Inclined-coil Ammeter. Referring 

 to Fig. 36, the current flows through the coil which is inclined 

 at an angle of about 45 to the spindle which carries the iron 



