26 



A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



By far the largest number of tests were made with a current of 1.25 

 amperes, which was selected for the study of the magnetic field described 

 in Part III of the report. In order to make this study it was necessary 

 to move the disk as far as possible toward the rear pole-face and thus 

 provide space in the air-gap between the front pole-face and the disk for 

 a flat bismuth spiral. It was deemed advisable, therefore, to test the 

 machine under these conditions in order to find if there was any marked 

 difference in the calibration test when the circular disk was somewhat 

 off center. The points obtained in this way are surrounded by circles 

 in the curve shown in fig. 13. It will be seen that they lie somewhat 

 above the curve, as was also the case with figs. 9 and 12. The reason 

 doubtless is that the magnetic field, at least near the edges of the poles, 

 is so non-uniform that the lines of induction intercepted by the disk are 

 somewhat denser when the latter is brought close to one pole-face. 



.02 3 



.022 



.021 



.020 



.019 



.018 



.01 7 



.016 



.015 



30 40 50 60 70 80 90 100 110 



Fig. 12. Calibration curve of ergometer II for magnetizing current of 1.50 amperes. 



Tests were also made with the magnet covering more of the copper 

 disk, i.e., pushed in about 2 cm. toward the hub. Accordingly, in fig. 13 

 we find a large number of points which may be classified under several 

 groupings. In the series of observations in which the magnet was pushed 

 farther over the copper disk, one might expect a somewhat smaller brake- 

 effect upon the copper disk; as a matter of fact, it was found that the 

 curve was shifted somewhat to the left, showing abnormally high values 

 of heat per revolution at low speeds. These points are indicated by 

 squares (c/. Part III). Since the chief use of the instrument, however, 

 is for a regular magnetizing current of 1.25 amperes, with the disk ex- 

 actly in the center of the air-gap and the periphery of the disk tangen- 



