Hi everyone, my Googling thus far hasn't yielded an answer, so I'm asking here.

The general equation for calculating the inductance of a coil is:

L = (N^2 * u * A) / l

Where:

L is the inductance

N is the number of turns of wire in the coil

u = the magnetic permeability of the core material, (air, iron, etc)

A is the cross sectional area of the core

l is the length of the coil winding

So, my question is this: What is l, the length of the winding, for a toroidal coil, if the winding doesn't take up the full length of the toroid?

If one only has a couple turns that are not spaced out to take up the full circumference of the coil, does one use the actual length of the coil, or the length of the circumference of the toroid?

And, follow up question, if one is using the length of the circumference of the toroid, does one use the inner circumference, the outer circumference, or the middle radius circumference?

I guess one could spread the windings out to take up the full space available, but if that isn't done, what is "l" ?

Thanks,

JC