MAGXKriC DKSUi.N OF KKLAYS 



51 



air path. A common case is that of a pormaiioiit magnet magnetized as 

 a separate part prior to assembly in a polar structure. In such ( ases, the 

 reluctance of the return path can be estimated as that of the external 

 field of a bar magnet. 



The r(>luctance of a bai- magnet is closely represented by the reluctance 

 of the same magnet in the form of a ring, in series with a reluctance 

 representing all the flux return paths. Values for this reluctance may be 

 assigned by measurements of magnetomotive forte to produce a given 

 flux in a ring sample and in a l)ar. The difference in magnetomotive force 

 gives a measure of the rcluctaiu e of the air path. It has been found by 



l(R 



Fig. 18 — Effective leakage reluctance of a bar magnet. 



Thompson and jMoss" that the reluctance per unit length of bar depends 

 on the shape of the bar cross-section, and is a function of the magnet 

 dimensions (/y/a , as indicated in Fig. 18. The reluctance so shown 

 includes all flux lines emanating from the bar, and so may be thought 

 of equally as leakage or effective reluctance. 



Ca.ses such as the bar magnet are difficult to estimate because of the 

 variable flux density along the length of magnetic material. Calculations 

 have been found possible only for the case of an ellipsoid, which is rarely 

 met in practice. 



Reluctance of a Solenoid 



Another special case is that of the air field of a coil, which has the 

 character shown in Fig. 19. I'liis type of field obtains in the initial flux 



