i IJ FLUX AND MAGNETOMOTIVE FORCE 9 



flux is found to be equal to 21 kilo-maxwells. The permanent winding 

 on the same ring must produce a flux of 65.1 kilolines at a current of 

 9.3 amperes. How many turns will be required? Ans. 2750. 



6. The Permeance of a Magnetic Path. In calculations per- 

 taining to the electric circuit it is convenient to deal with the recip- 

 rocals of resistances when conductors are connected in parallel. 

 The reciprocal of a resistance is called a conductance and is meas- 

 ured in mhos if resistance is measured in ohms. Similarly, a die- 



ic is characterized sometimes by its elastance, at other 

 by the reciprocal of its elastance, which is called permittance. 

 \Vheii permittance is measured in farads, elastance is measured in 

 darafs (see the chapter on the Electrostatic Circuit in the author's 

 it). 



Analogously, when two or more magnetic paths are in parallel 

 it is convenient to use the reciprocals of the reluctances. The 

 reciprocal "f the reluctance of a magnetic path is called its per- 

 meance; eq. (1) becomes then 



(? $ 

 where V ' 



/" T"*"""*^* / w * f "*' " ' if 



(3) 



A script (P is used for permeance in order to avoid coni'iisini: it 

 with pov, the unit of permeance corresponding to tin- 



he author proposes the name perm. A magnetic path has a 

 eance of one perm when one maxwell of flux is produced for 

 each ampere-turn of magnetomotive force applied along the path. 



The unit " perm " has been in use among electrical desi^ 



for some time, although no name has l>een given to it. Notably 



Mr. II. M has used it extennvcly in his writings, 



in the calculation of the inductance of windings. He speaks of 



.aietic lines per ;nit length" (of the 



Idcil part of a coil). This is equivalent to perms per unit 



h. 



In the ampere-ohm .-vstem the iel unit 



imeance is the henry. 1 Therefore, ji j n eq. (2) M is measured 



IIH and in webers, (P is in henrys, and no new unit 



for permeance is necessary. In this case the reluctance (ft in eqs. 



though the henry is denned at tho unit ..f indueUnee, it is shown in 

 ..w that permeance and inductance are phyrically of the Mine 

 dtoeMJoni and hence meaturembie in the une unit ^ 



