AMMETERS 



341 



FIG. 233.- 



The prin- 

 ciple of the 

 galvanome- 

 ter. 



A coil of wire free to rotate is suspended by a wire be- 

 tween the poles of a strong magnet ; when a current is sent 

 through the coil, the coil becomes a magnet and turns so that 

 its faces will be towards the poles of the perma- 

 nent magnet. But as the coil turns, the suspend- 

 ing wire becomes twisted and hinders the turning. 

 For this reason, the coil can turn only until the 

 motion caused by the current is balanced by the 

 twist of the suspending wire. But the stronger 

 the current through the coil, the stronger will be 

 the force tending to rotate the coil, and hence 

 the less effective will be the hindrance of the 

 twisting string. As a consequence, the coil 

 swings farther than before ; that is, the greater 

 the current, the farther the swing. Usually a 

 delicate pointer is attached to the movable coil 

 and rotates freely with it, so that the swing 

 of the pointer indicates the relative values of the current. 

 If the source of the current is a gravity cell, the swing of 

 the needle is only two thirds as great as when a dry cell 

 is used, indicating that the dry cell furnishes about i J t;mes 

 as much current as a gravity cell. 1 



314. Ammeters. A galvanometer does not measure the 

 current, but merely indicates the relative strength of different 

 currents. But it is desirable at times to measure a current 

 in units. Instruments for measuring the strength of currents 

 in units are called ammeters, and the common form makes 

 use of a galvanometer. 



A current is sent through a movable coil (the field magnet 

 and coil are inclosed in the case) (Fig. 234), and the magnetic 

 field thus developed causes the coil to turn, and the pointer 



1 In the illustration, soft iron has been placed inside the coil to increase the 



effect. 



