8 THE DIRECT-CURRENT MOTOR CH. I 



commutator bar provides at least two surface conductors, 

 and the total number of surface conductors is at least 

 twice as many as the number of bars. 



Example 7. A railway motor has 100 bars on the 

 commutator with four turns per bar; the armature is 

 drum wound. A is 800. 



Example 8. A motor has a ring-wound armature, 

 120 commutator bars, and 8 turns per bar. The number 

 of surface conductors is 960. 



Equation 5 shows that we can find the induction 

 factor of a dynamo by observing the current in the 

 armature and the corresponding torque on the shaft. 



A lever is bolted to the shaft of the dynamo, and the 

 pull at a measured distance from the centre observed by a 

 spring balance, which should be provided with a metal 

 stirrup to go under the lever. The balance should be 

 raised until the lever is horizontal, and the pull then 

 read. 



We shall find that we get a small reading on the 

 scale when there is no current in either magnets or 

 armature. This is due to the friction of the bearings and 

 brushes ; we may assume it to be constant throughout the 

 experiment. On exciting the magnets we shall generally 

 find that the pull is slightly increased without having any 

 current in the armature ; this will be explained later on, 

 but we may state briefly that a definite torque is required 

 to turn a mass of iron in a magnetic field, the amount of 

 the torque depending on the quality of the iron, the 

 intensity of the field, and the volume of iron turned ; if 

 the current exciting the magnets be kept constant, this 

 torque, which we shall call the ' hysteresis torque,' will be 

 constant. 



