42 A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



INFLUENCE OF TEMPERATURE ON THE CONSTANCY 

 OF THE BICYCLE ERGOMETER. 



From what has preceded, it is clear that the rate of heat production 

 varies inversely as the resistance of the rotating disk, and hence that 

 the heat per revolution varies in the same manner. Over the usual range 

 of room temperatures, it may be assumed that the same expenditure of 

 energy in the disk raises its temperature to the same extent above its 

 surroundings. If the ergometer is used outside the calorimeter in a room 

 at the same temperature as that inside the calorimeter during calibration, 

 the results of the calibrations can be applied without correction, provided 

 the circulation of air is approximately the same in the two cases. But if, 

 for example, an accuracy of 2 per cent in the energy measured is desired, 

 then, since the temperature coefficient of copper is approximately 0.004, 

 a temperature correction will have to be applied if the temperature of the 

 room differs by more than 5 C. from the mean temperature inside the 

 calorimeter during calibration. In general, during the work that has 

 been done thus far with the ergometer, no such correction has been neces- 

 sary. The highest observed temperature of the disk (see Part II) was 

 43 C. at a pedal speed of 120 revolutions per mmute, the room tempera- 

 ture being 20 C. It was to be expected that as the speed increased the 

 maximum temperature would occur at a higher speed than the maximum 

 value of the heat per revolution, since the maximum temperature depends 

 on the heat per second, i.e., it is proportional to the heat per revolution 

 multiplied by the speed. In using the ergometer for accurate quantita- 

 tive measurements, care should always be taken to maintain each speed 

 long enough for the temperature of the copper disk to reach a sufficiently 

 steady state. For practical purposes, this precaution is seldom necessary. 



THE DESIGN OF ELECTRIC BRAKES. 



In conclusion, we will summarize briefly the general principles that 

 ought to be considered in the design of apparatus employing electro- 

 magnetic damping, particularly with reference to the demagnetizing 

 effects of the eddy currents. We shall base our deductions in part on the 

 equation 



derived from our fundamental assumptions on p. 32. </> is the impressed 

 flux when the disk is stationary, co the angular velocity, a- the specific 

 resistance, and k a constant. 



(a) Material of disk. For the strongest effects soft iron or soft steel 

 may be used. The resistance is of course comparatively high, but the 

 magnetic induction will be very large, and the heating due to hysteresis 

 will be added to that from the eddy currents. The magnetic reaction 

 from an iron disk must be very large, as was indeed shown by Hertz in 

 the paper already cited. Copper and aluminum are probably the most 



