84 THK PlIfKCT-rrHli'KNT M()T<)|; en. | 



should know that the frictional losses in the combination 

 were equal to 20 inrh-potinds. 



Since the current from the line is nothing we might 

 disconnect the line altogether without making any 

 change in the conditions. We have here Hopkin- 

 son's method of testing dynamos. See 'The 

 Electrician,' March 12, 1886, and the 'Proceedings of 

 the Institution of Civil Engineers/ Vol. LXXX1I1. p. 

 235. This method consists in mechanically coupling two 

 similar dynamos, their common shaft being provided with 

 a pulley. The two machines are driven by a belt, the 

 power transmitted being measured by a dynamometer. 



Suppose that a test of a generator has to be made, the 

 conditions being that it has to run at a given speed and 

 give out a certain current at a certain terminal tension. 

 First we must arrange to drive the combination at the given 

 speed and to keep it constant throughout the test. Then 

 we must adjust the induction factor of the motor so that 

 when the combination is running at the specified speed, 

 we shall have the proper current passing in the two 

 dynamos. When this is the case, the belt and the motor 

 together will be driving the generator, and the power 

 transmitted by the belt will represent the heat and torque 

 losses in the two machines. Knowing the terminal ten- 

 sion, the resistance of the generator, and the speed n at 

 which it has to run. we know the induction factor, 

 call this M (l . Then the difference between the two induced 

 tensions is the effective tension and equal to the product 

 of the current into the sum of the two resistances, so 

 that M b , the induction factor of the motor, is given by 



^/, = .l/, ( - c ( /l) " +Zi> ") ........... (43). 



n 



