MECHANICAL REVERSAL OF THE CURRENT 335 



When the current which enters at B flows through the 

 wire, the coil rotates, tending to set itself so that its north 

 face is opposite the south face of the magnet. If, when the 

 helix has just reached this position, the current is reversed 

 entering at B' instead of B the poles of the coil are ex- 

 changed ; the rotation, therefore, does not cease, but continues 

 for another half turn. Proper reversals of the current are 

 accompanied by continuous motion, and since the disk and 

 shaft rotate with the coil, there is continuous rotation. 



If a wheel is attached to the rotating shaft, weights can be 

 lifted, and if a belt is attached to the wheel, the motion of 

 the rotating helix can be transferred to machinery for prac- 

 tical use. 



The rotating coil is usually spoken of as the armature, and 

 the large magnet as the field magnet. 



310. Mechanical Reversal of the Current. The Com- 

 mutator. It is not possible by hand to reverse the current 

 with sufficient rapidity and precision to insure uninterrupted 

 rotation; moreover, the physical exertion of such frequent 

 reversals is considerable. Hence, some mechanical device 

 for periodically reversing the current is necessary, if the 

 motor is to be of com- 

 mercial value. 



The mechanical re- 

 versal of the current is 

 accomplished by the 

 use of the commutator, 

 which is a metal ring 

 split into halves, well 



insulated from each FIG..**.- The commutator, 



other and from the 



shaft. To each half of this ring is attached one of the ends of 

 the armature wire. The brushes which carry the current are set 



