MAGNETIC EFFECTS OF ELECTRIC CURRENTS 235 



^V poles or two S poles are used near the rotating coil. Why? 

 The necessary parts of a direct current motor are a suitable 

 magnetic field, a coil for rotation (usually called the armature), 

 and a device (usually called the commutator) to send the 

 current into the armature at the proper instant and in the 

 correct direction. 



240. Different kinds of motors. Permanent magnets are not 

 adapted for use in motors. A modification must be made in 

 order that the magnetic field may be furnished by a stronger 

 electromagnet, as is done in all practical motors. In figure 119 

 the bar magnets have been swung outward from the magnet. 

 A coil of wire wound upon a suitable iron frame is placed 

 near the armature. The motor may now be operated: first, 

 by using a different source of current for the armature and 

 electromagnetic field ; second, by connecting the armature 

 and field (the terminals of the coil) in series with a source 

 of current ; and third, by arranging the armature and field 

 circuit in parallel (shunt) with a common source of current. 



The first method is not suitable for practical motors ; the 

 second illustrates the series motor, which is used in electric 

 vehicles and at times on street cars ; and the third represents 

 the so-called "shunt" motor, which has extensive use. An 

 additional type of direct-current motor, called the compound 

 motor, is a combination of the series and the shunt motor. 

 In addition to the direct-current motors there arc; several 

 different types of machines that are designed to operate upon 

 an alternating-current circuit. 



241. Domestic and commercial uses of motors. Electricity 

 provides the most convenient means of transmitting and using 

 energy. The electric motor may have perfect control, relia- 

 bility of operation, range of distribution, cleanliness, and less 

 danger of accidents than other sources of energy. Motors 

 are used in the home to operate washing-machines, vacuum 

 cleaners, meat choppers, ice machines, fans, elevators, pumps, 

 and ice-cream freezers. 



