24 



A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



With ergometer I the magnetizing current ranged from 0.70 ampere 

 to 1.25 amperes, but inasmuch as the winding of the magnet in ergometer 

 II was somewhat different, the current ranged from 0.95 ampere to 1.50 

 amperes in the calibration tests of this ergometer. It was planned to 

 secure calibrations of the ergometer at each current with variations in 

 speed ranging from approximately 50 to 120 revolutions of the pedals 

 per minute. For a given speed, the highest values of heat per revolution 

 were obviously found with the largest magnetizing current, namely, 1.50 

 amperes. As a matter of fact, however, the experiments of November 

 4 and 6 show that with less current (1.35 amperes) through the field- 

 coils but with a low speed, the heat per revolution was exactly the same 

 as with a current of 1.50 amperes and with twice the number of revolu- 

 tions, namely, 118 revolutions per minute. It is impossible, however, 

 to analyze satisfactorily the varying conditions without recourse to a 

 series of curves plotted for each intensity of magnetizing current. 



.015 

 .014 



013 



012 



11 50 60 70 80 90 100 110 120 



Fig. 9. Calibration curve of ergometer II for magnetizing current of 0.95 ampere. 



x ^s 



.018 

 .017 

 .016 

 .015 

 .014 



t 013 



Fig. 10. Calibration curve 



x x 



x X. 



N, x 



x >y 



X 



70 8O 90 IOO 110 120 



of ergometer II for magnetizing current of 1.10 amperes- 



Beginning with the lowest current, namely, 0.95 ampere, we find that 

 the values all lie fairly close to the curve (see fig. 9). Two of the obser- 

 vations shown on this curve were made when the disk was rotating very 

 close to the rear pole, leaving a wide air-gap on the other side. These 

 two values, which are indicated by small circles, lie approximately on the 



