138 COMMERCIAL ELECTRICAL TESTING 



the motor a mark should be made at M, and a pointer, 

 P, attached to the lever as shown. The lever arm is 

 then raised until the distance XT=YS, and the pointer 

 set so that it is on mark M. If the weight of the lever 

 alone is not sufficient to overcome the friction of the 

 bearings and turn the rotor round till the end of the lever 

 touches the floor, attach a weight at W. Now if the 

 spring balance H is pulled upwards until the pointer 

 P is on mark M, XV will be parallel to 75, and the pull 

 X will make an angle of 90 deg. with the center of the 

 shaft the position in which all readings must be taken. 



T o 



Fig. 41. Measurement of Torque by Means of Spring Balance 



Open all switches on the dynamometer board to elimi- 

 nate the residual magnetism of the alternator, raise the 

 lever arm by pulling vertically on the spring balance 

 till the pointer passes the mark M, and at the instant 

 of passing take a reading of the spring balance. Call 

 this reading W-\-F. Let the lever be raised until the 

 pointer is some distance beyond M\ then lower the 

 spring balance and let gravity pull the lever toward 

 the floor, reading the balance when the pointer passes the 

 mark. Call this reading W F. To get good readings, 

 the lever arm should be moved rather slowly, but 

 steadily, and with as nearly constant speed as possible, 

 a reading of the balance being taken every time the 

 pointer passes the mark. As a check, three or four read- 

 ings should be taken as described above. 



