So, I was wondering, what is the electrical model of mutual inductance?

You understand self-inductance, yes? Ignoring winding resistances, given

a two winding transformer, if you apply a current to one winding (or to a

single winding inductor), the voltage across the inductor is L di/dt. In

the case of a sinusoidal AC current i of frequency f in the inductor, the

voltage e across the inductor is

e = L*2*pi*f*i, or L = e/(2*pi*f*i). L is the ratio of the voltage across

the winding to the rate of change of the current in the winding.

Roughly, it's a relationship between the voltage across a winding and the

current producing it.

In the case of mutual inductance, it's the ratio of the voltage across a

winding to the rate of change of current in *another* winding.

It's explained in my physics textbook, with no electrical significance as

far as I can tell.

Leakage inductance is far simpler to deal with, but I noticed that my

textbook doesn't mention it and internet sites deal with cored examples.

Air core transformers have leakage inductance too!!

Purpose: wondering what the LL of coupled, concentric solenoids with

different diameters is.

Frederick Grover's book on inductance calculations

http://www.amazon.com/Inductance-Ca...5449707?ie=UTF8&s=books&qid=1173230870&sr=1-1
will tell you how to calculate the self-inductance of the solenoids and

also their mutual inductance. From this you can calculate the leakage

inductance. However, I will tell you that it will probably be easier to

build them and measure.