CALIBRATION TESTS 



27 



tial to the upper edge of the pole-face of the magnet, it seemed undesirable 

 to make further calibrations of this instrument under these peculiar 

 conditions, which were necessitated only by the study of the magnetic 

 field. We have, therefore, chiefly to consider the observations made with 

 the disk in the regular position. The heavy line 1 plotted curve represents 

 all observations with the disk and magnet in their original positions. 

 Here again we find with low speeds the low heat per revolution a fairly 

 constant heat per revolution with a speed between 60 to 90, and a fall 

 in heat per revolution as the speed increases beyond this. 



.022 



.021 



.020 



.01 9 



o 



o 



;c 

 ^-"" ^ o 

 7^ ^*"""^ ^K. 



r \ x \. 

 \ ;c 



.01 8 

 .01 7 

 .016 

 .015 



.014 



.013 



20 30 40 50 60 70 80 90 lOO 110 120 



Fig. 13. Calibration curves of ergometer II for magnetizing current of 1.25 amperes. 

 Black crosses: Disk in normal position between poles of magnet; curve in heavy line is based upon these. 

 Circles: Disk close to rear pole, air-gap on other side widened. 

 Squares: Poles moved 2 cm. in toward center of disk. 



Curve in light line represents values of ft)(f> 2 (see p. 37). 



To show their similarity the curves corresponding to the five magnet- 

 izing currents, 0.95, 1.10, 1.25, 1.35, and 1.50 amperes have been replot- 

 ted on one diagram (see fig. 14). This series of curves is strikingly simi- 

 lar to those found with ergometer I when calibrated in June and July 

 of 1911, and indicates that the instruments are essentially alike in their 

 mechanical and electrical features. The special feature to be noted here 

 is that the curves show uniformly a low heat per revolution with a low 

 speed, nearly constant heat per revolution between approximately 60 to 

 90 revolutions per minute, and a rapidly falling heat per revolution at 

 high speeds. Since practically all experiments are made with bicycle 

 riders at speeds between 60 to 80, it may be stated again that, in general, 



1 The lighter lined curve is discussed in Part III, p. 37. 



