MAGNETIC REACTIONS 43 



widely used metals on account of their low specific resistance. Aluminum 

 has also a high specific heat to recommend it. If a weaker effect is de- 

 sired, an alloy of high resistance may be used. Other things being equal, 

 the demagnetizing effect of the eddy currents will be greatest for an iron 

 disk, with copper and the other non-magnetic metals following in the 

 order of their specific resistances. But for the same expenditure of power 

 the demagnetizing effect will be practically independent of the material 

 of the disk. 



(6) Thickness of disk. With the same magnetic flux, the intensity of 

 the induced currents and also the heat will vary directly as the thickness. 

 Since, according to Hertz, the currents in the interior of a thick disk lag 

 more than those on the surface, it follows from equation (2) that the 

 demagnetizing effect will increase at a more rapid rate than will the thick- 

 ness of the disk. 



In conjunction with the magnetic field and gear ratio employed, the 

 particular thickness of disk used in these ergometers fortunately was just 

 such as to produce a nearly constant heat per revolution over the range 

 of speeds commonly used by riders. 



(c) Diameter of disk. This is probably of small consequence as long 

 as the magnet pole covers only a small part of the surface of the disk. 

 The essential factor is the linear velocity of the metal under the pole. 



(d) Linear velocity. The expenditure of power increases of course 

 with increasing velocity. On the other hand, equation (6) shows that 

 the counter-flux increases also, tending toward <f) as a limit. Hence 

 in order to minimize the demagnetizing action for a given amount of 

 power to be absorbed, it is best to use a large magnetic flux and a low 

 speed. 



(e) Size and shape of pole-piece. The most important quantity is the 

 width, measured in a direction tangential to the disk. The current paths 

 may be regarded as consisting of two parts, one lying in a radial direction 

 under the pole, in which the currents are induced, and the other consist- 

 ing of the remainder of the disk, in which the circuits are completed. If 

 the polar area is small in comparison with the area of the disk, it follows 

 that the first portion mentioned will contain most of the ohmic resistance 

 of the circuits, since the lines of flow are here very constricted. Hence 

 the resistance may be assumed to be inversely proportional to the width 

 of pole. If now the same total flux be spread out over a pole-face n times 

 as wide, the total current will remain unchanged, while the production of 



heat and therefore the consumption of energy will be - as great. On 



the other hand, if the magnetic induction remains constant, so that the 

 total flux varies directly as the width of pole, the consumption of energy 

 will also vary in the same manner. 



The demagnetizing effect will probably be somewhat less with a broad 

 pole, since the same angular lag will then not bring the demagnetizing 



