44 A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



system of current loops so directly under the pole. This is the case in 

 the damping disk of watt-hour meters, which in addition to broad pole- 

 faces employ thin disks and low speeds, thereby reducing the demagnet- 

 izing factor to a minimum. 



Lengthening the pole-face in a radial direction will, by reasoning 

 analogous to the preceding, cause a proportionate increase in the ex- 

 penditure of energy if the flux density is kept constant, and a decrease 

 in the same ratio if the total flux is constant. 



(/) Intensity of magnetic field. The consumption of energy varies 

 as the square of the flux density. The percentage of demagnetization 

 from the eddy currents is a constant for the same speed, independent of 

 the field intensity. This explains why the maxima of the calibration 

 curves in figs. 8 and 14 all occur at practically the same speed, whatever 

 the current in the electro-magnet. 



(g) Reluctance of the magnetic circuit. To insure a "stiff" field, re- 

 sisting the demagnetizing action of the eddy currents, it would be advan- 

 tageous to use a magnetic circuit of relatively large reluctance and large 

 magnetomotive force, with strongly saturated poles. Crowding of the 

 flux in the neighborhood of the trailing edge of the pole could be reduced 

 by widening the air-gap on that side of the magnet, or by using split pole- 

 pieces, like those in the Lundell generators. By inserting a variable 

 air-gap in the magnetic circuit, the maximum of the calibration curve 

 could probably be shifted to the right or left. 



(h) Location of magnet poles. These should be far enough from the 

 outer edge of the disk to minimize magnetic leakage around the edge. 

 The entire magnet should be shaped in such a way as to reduce the leak- 

 age, especially in the neighborhood of the poles. This requirement is 

 met, for example, in the permanent magnets of watt-hour meters. It is 

 true that our calibration curves (fig. 13) do not show any less evidence of 

 demagnetization when the poles are pushed 2 cm. nearer to the center of 

 the disk, but this is because there was still considerable opportunity for 

 magnetic leakage, owing to the construction of the magnet. 



Thus on the whole it will be seen that, for maximum expenditure of 

 energy, it is advantageous to use small magnet poles, while to minimize 

 the magnetic reaction the poles should be broad. The best compro- 

 mise between these opposing factors can only be reached by experiment. 

 In any case, the magnetic field should be as intense as possible. 



