336 MAGNETS AND CURRENTS 



on opposite sides of the ring and do not rotate. As armature, 

 commutator, and shaft rotate, the brushes connect first with 

 one segment of the commutator and then with the other. 

 Since the circuit is arranged so that the current always 

 enters the commutator through the brush B, the flow of the 

 current into the coil is always through the segment in contact 

 with B ; but the segment in contact with B changes at every 

 half turn of the coil, and hence the direction of the current 

 through the coil changes periodically. As a result the coil 

 rotates continuously, and produces motion so long as current 

 is supplied from without. 



311. The Practical Motor. A motor constructed in accord- 

 ance with Section 309 would be of little value in practical 

 everyday affairs ; its armature rotates too slowly and with 

 too little force. If a motor is to be of real service, its arma- 

 ture must rotate with sufficient strength to impart motion to 

 the wheels of trolley cars and mills, to drive electric fans, and 

 to set into activity' many other forms of machinery. 



The strength of a motor may be increased by replacing the 

 singly coiled armature by one closely wound on an iron 

 core; in some armatures there are thousands of turns of 

 wire. The presence of soft iron within the armature (Section 

 296) causes greater attraction between the armature and the 

 outside magnet, and hence greater force of motion. The 

 magnetic strength of the field magnet influences greatly the 

 speed of the armature ; the stronger the field magnet the greater 

 the motion, so electricians make every effort to strengthen 

 their field magnets. The strongest known magnets are elec- 

 tromagnets, which, as we have seen, are merely coils of wire 

 wound on an iron core. For this reason, the field magnet is 

 usually an electromagnet. 



When very powerful motors are necessary, the field mag- 

 net is so arranged that it has four or more poles instead of 



