THE MAGNETIC CIRCUIT ELECTROMAGNETS 25 



surfaces, the lines of flux are supposed to follow the path indicated 

 in Fig. 8. Let I stand, as before, for the length measured per- 

 pendicularly to the plane of the section shown, then, 



= 



Jo 



I . irt 

 = - log e - 



7T &e S 



(10) 



FIG. 8. Permeance between surfaces in same plane. 



Case (d). Iron-clad Cylindrical Magnet. Fig. 9 shows a 

 section through a circular magnet such as might be used for 

 lifting purposes. The exciting coil is supposed to occupy a 

 comparatively small portion of the total depth, and in order to 

 calculate the total flux between the inner core and the outer 

 cylinder forming the return path for the useful flux we may 

 consider the reluctance of the air path as being made up of a 

 number of concentric cylindrical shells of height h and thickness 

 dx. Thus, 



J R dx 

 rlirxh 

 R 



(H) 



When the radial depth of the winding space (R - r) is not 





The reciprocal of this quantity is the permeance, whence, 



