Page 70 - Basic Electrical Engineering

Therefore,

Again,

where

Remember that reluctance is the inverse of the permeability. Low reluctance
will give rise to a high value of inductance. That is why in order to produce
high value inductance, the number of turns should be high and the reluctance

to the flux path should be low. The core should be made of high permeability
material like iron.

Again,

or,

LI = Nϕ

Considering a small increase of i producing a small increase in ϕ as dϕ

Ldi = Ndϕ