Electromagnetic Induction. 



343 



The first set of curves of constant induction are given in 

 fig. 2, Plate X. Curve 20 and the point of maximum nega- 

 tive induction could not be found experimentally; they are 

 therefore only given as near approximations. The rectangular 

 space near the origin shows the region within which the auxi- 

 liary coils C and D (see p. 338) had to be employed. These 

 curves represent the contour-lines of one quadrant of the sur- 

 face above described, and show at what distance from the 

 origin any curve cuts the axis of y. Knowing this, and know- 

 ing also the relative value of M or of z for each curve, 

 we obtain a set of values for the coordinates of a curve which 

 may be viewed as a section of the surface in question in a 

 plane containing the axes of y and z. These coordinates are 

 given in Table IV.; the curve plotted by means of them is 

 given in fig. 3, PI. XL; but the dotted part of it is only ap- 

 proximate. 



Table IV. 



y- 



z. 



y- 



y'- 



z. 



2-6 



39-55 



6-15 



26-0 



- 0125 



307 



32-0 



617 



20-22 



- 0-25 



4-38 



16-0 



6-19 



16-27 



- 05 



518 



8-0 



6-23 



13-305 



- 1-0 



5-64 



40 



633 



11085 



- 20 



5-88 



20 



6-58 



9-44 



- 40 



5-99 



10 



702 



8-48 



- 8-0 



6-06 



05 



7-45 



80 



-16-0 



611 



025 



7-7 



7-7 



-320 



6-13 



0125 









6-14 



o-o 









In order to give a better idea of the symmetry of the curves 

 of constant induction than can be got from fig. 2, the curves 

 in that figure have been repeated in the other quadrants, and 

 are given in fig. 4, PI. XL, for the whole of the magnetic field 

 of the primary coil. Now every thing is symmetrical rela- 

 tively to the axis of x, and each curve represents a section of 

 a surface of revolution about that axis. Hence, if the curves 

 are supposed to revolve round the axis of x, a number of sur- 

 faces of revolution will be generated, each of which will be a 

 surface of constant induction, the surfaces of positive induction 

 being separated from those of negative induction by the sur- 

 face of no induction. The positive surfaces may be described 

 as shells which enclose one another, and each of which turns 

 inwards, closing up round the axis of x on each side of the 

 origin. The zero-surface, which divides the positive from the 

 negative surfaces, instead of closing up, may be supposed to 



2D 2 



