36 A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



the other curves agree among themselves to about the same degree of 

 closeness as these. Owing to the unsatisfactory character of the obser- 

 vations in the middle of the field when the disk was in motion, but little 

 weight was placed on these data, and the curves are accordingly shown 

 as broken lines in this region. 



Since the bismuth wire was coiled in a spiral about 17 mm. in diameter, 

 it is clear that these curves can not show accurately the precise form of 

 the magnetic field. A simple consideration shows that if the curves 

 could be drawn with precision they would slope more steeply than the 

 curves here drawn; they would then cross the lines G G' at points higher 

 up, and the maxima would all be higher. Still, crude as they are, they 

 show clearly the reaction of the eddy currents in the disk. 



The curve obtained with the disk stationary (speed 0), is quite sym- 

 metrical, showing slight maxima close to the edges of the poles. As 

 the speed increases, the distortion of the magnetic field and the marked 

 decrease in flux at high speeds are very evident. From the curve for 

 speed 364, it might be inferred that here the induced current is confined 

 entirely to a narrow path close to the trailing edge of the pole-face. That 

 this is the case will be shown later. 



Since the ordinates of the curves for speeds 36, 195, and 364 represent 

 the resultant induction through the disk, it is evident that the algebraic 

 difference between these ordinates and those for speed must be a 

 measure of the magnetic field that would be produced by the induced 

 currents alone. These differences are plotted in fine lines. Negative 

 ordinates signify a component opposing the flux from the electro-magnet. 

 The most striking feature of these curves is the very pronounced demag- 

 netizing field produced in the disk at high speeds. The points where the 

 curves cross the axis of absciss oe show that the displacement of the cur- 

 rents in the direction of rotation increases with the speed (eq. (1)), though 

 at a lower rate. It is presumably near these points that the induced 

 currents attain their maximum values. 



A few observations were made with the bismuth spiral in other posi- 

 tions. The induction was found to be practically uniform when the 

 spiral was moved in a radial direction, except close to the outermost edge 

 of the magnetic field near the circumference of the disk, for example at 

 the point P in fig. 16. Here the flux density was found to increase with 

 increasing speed, as would be expected, for the demagnetizing effect of 

 the currents must lead partly to a diminution in the total flux around the 

 magnetic circuit, and partly to increased leakage around the outer edge 

 of the disk. Indeed, the currents along the edge of the disk on the 

 side approaching the magnet flow in such a direction as to bend the lines 

 of magnetic induction outward around the edge of the disk. 



When the spiral was laid flat against the side of the magnet pole, 

 with its plane perpendicular to the disk, it showed a decrease of about 

 30 per cent in magnetic induction on the "leading" side, while on the 



