Consider section i, one of the inner sections. The other two will be special case because of the boundary conditions.
Each of the terms in the Equation 1.5 will be now approximated:
Where
Therefore
The other terms can be similarly approximated.
Please note that the values of the absolute permeability are known per
section and thus given at the mid point 
for 
.
The harmonic average is recommended to average these values.
Now we can write a discrete approximation of Equation 1.5.
Consider inner rod-section 
,
Which can be rewritten as
