10 THE MAGNETIC CIRCUIT [ART. 6 



(1) and (3) is in henrys" 1 ; or spelling the word henry backwards, 

 as in the case of mho and daraf, the natural unit of reluctance in 

 the ampere-ohm system gets the euphonious name of yrneh (to be 

 pronounced earney). 



Since, however, the maxwell is used almost exclusively as the 

 unit of flux, it seems advisable to introduce the rel and the perm 

 as units directly related to it. Should engineers gradually feel 

 inclined to use the weber and its submultiples as the units of flux, 

 then the henry, the yrneh, and their multiples and submultiples 

 would naturally be used as the corresponding units of permeance 

 and reluctance. 



We have, therefore, the two following systems of units for 

 reluctance and permeance, according to whether the maxwell or 

 the weber is used for the unit of flux (one ampere-turn being the 

 unit of m.m.f. in both cases): 



One perm = 10~ 8 henry ; one rel = 10 8 yrnehs. 



Prob. 7. What is the permeance of the magnetic circuit in prob. 4? 



Ans. 0.0483 perm. =4.83 X 10~ 10 henry. 

 Prob. 8. What is the permeance of the ring in prob. 6? 



Ans. 2.545 perm. = 0.02545 microhenry. 



Prob. 9. How many ampere-turns are required to maintain a flux 

 of 2.7 megalines through a permeance of 750 perms? Ans. 3600. 



6. Reluctivity and Permeability. The reluctance of a magnetic 

 path varies with the dimensions of the path according to the same 

 law as the resistance of an electric conductor or the elastance of a 

 dielectric. That is to say, the reluctance is directly proportional 

 to the average length of the lines of force and is inversely propor- 

 tional to the cross-section of the path. This relationship can be 

 verified by measurements on rings of different dimensions (Fig. 1). 

 When the diameter of the ring is increased twice, keeping the same 

 cross-section, the length of the path of the flux is also increased 

 twice. Experiment shows that the new ring requires twice as 

 many ampere-turns as the first one for the same flux; or, only one- 

 half of the flux is produced with the same number of ampere- 



