80 
DIREC'T-CURRENT DYNAMOS AND MOTORS. 
Flux 
Permeance 
~~ 
Magnetomotive Force = 
But the permeance, that is, the magnetic conductance, of 
a path may be expressed by the permeability of the 
material and the dimensions of the magnetic path, just as 
the electric conductance of a body is its conductivity 
times its area, divided by its length; consequently we 
have: 
aff Area 
Permeance = Permeability x Lengit 
Inserting this expression of the permeance into (88), we 
obtain 
Flux x Length 
Permeability x Area 
Magnetomotive Force = 
and since the quotient of flux by area is the magnetic flux 
‘ density B, we have 
, _ Density 
Magnetomotive Force = Pactacauian x Length, 
or M. MF. = rae eee mele (39) 
From (39) the magnetomotive force is obtained in gil- 
berts, if the density B is expressed in lines per square 
centimetre and the length ZL, in centimetres; hence, ac- 
cording to Par. 34, Book 15: 
Ampere-turns = .796 x gtlberts 
= "Og x Density 
Permeability Leng 
or in symbols, practically: 
ae as . et eee (40) 
- Ifthe length of the path is given in inches, and the flux 
density in lines per square inch, we have: 
