WHAT MAKES THE TROLLEY CAR GO. 



325 



that the current, which starts with an infinitesimal strength, soon 

 rises to the maximum capacity of the machine. 



The motor shown in Eig. 10 does not in any way resemble an 

 electric railway motor, nevertheless the principle of action is pre- 

 cisely the same in both. The design of a machine of any kind has 

 to conform to the practical requirements, and this is true of rail- 

 way motors, just as it is true of printing presses, sawmills, or any 

 other mechanism. A railway motor must be designed to run at 



Fig. 15. — External View of Electric Railway Motor mounted upon Car-Wheel Axle. 



a comparatively slow speed and to develop a strong rotative force, 

 or torque, as it is technically called. It must also be so constructed 

 that it will not be injured if covered with mud and water. It must 

 be compact, strong, and light, and capable of withstanding a severe 

 strain without giving out. To render the machine water- and mud- 

 proof it is formed with an outer iron shell, which entirely incases 

 the internal parts. The first railway motors were not inclosed, 

 and the result was that they frequently came to grief from the 

 effects of a shower of mud. When the modern inclosed type of 

 motor, which is called the iron-clad type, first made its appearance 

 it was frequently spoken of as the clam-shell type, and the name 

 is not altogether inappropriate, for while the outside may be cov- 

 ered with mud to such an extent as to entirely obliterate the de- 

 sign, the interior will remain perfectly clean and dry, and there- 

 fore its effectiveness will not be impaired. 



To enable the motor to give a strong torque and run at a slow 

 speed the number of poles in the field and armature is increased. 

 The design of Fig. 10 has two poles in the field and two in the 

 armature, and is what is known as the bipolar type. Machines 

 having more than two poles in each part are called multipolar 

 machines. The number of poles can be increased by pairs, but 

 not by a single pole — that is, we can have four, six, eight, or 

 any other even number of poles', but not five, seven, or any odd 

 number. This is owing to the fact that there must always be 



