Hi:

To calculate mutual inductance of two wires, people refer to
the formula in the papaer provided by G. A. Campbell
(Physics Review Vol 5 pp452, 1915, available at
http://prola.aps.org/pdf/PR/v5/i6/p452_1 or send a e-mail to me
and I'll send you a copy)

It's a nice piece of work. The book of F. W. Grover
(Inductance Calculation (1962)) used his formula as well.

But I have a question on the case if the common perpendicular
Pp that is within one or two wires. They both said the formula
is for 'any' case, but I don't see how to apply it to these cases:

case 1: two wires, one lays over the other, making a 'X', any angle,
not 90 degree, not necessary centered. You raise the top one to a
distance.

case 2: two wires making a 'T' but with any angle, not 90 degree.
You raise one of the two to a distance.

If you know how to use his formula into above two cases, please
advise me. Thank you in advance.

Jian