OF ARTS AND SCIENCES. 



495 



closing the circuit of the dynamo through greater or less resistance, the 

 work done, and consequently the power absorbed by the brake, could 

 be varied. 



In testing small motors great care must be taken in measuring the 

 speed accurately. In these tests the speed indicator from a small siren 

 was used. This was connected to the motor by an endless band passing 

 from the pulley on the motor shaft to a pulley of the same diameter on 

 the siren. The power required to run this indicator was excessively 

 slight, and in computing the work done was neglected. The slip of 

 the belt was negligible as shown by numerous tests. With the Magneto 

 and Gramme "a petite lumiere" a continuously recording engine counter 

 was used connected to the motor by means of a flexible spiral spring. 



The following table gives in parallel columns the name of the motor 

 tested, the current in amperes, the electromotive force in volts at the 

 motor terminals, the activity in horse-power, the maximum efficiency, 

 and the speed for maximum efficiency in revolutions per minute. 



TABLE I. 



The table below gives the tests in full made on the model Edison 

 dynamo, and illustrates very well the relation of speed and etficiency. 



