one for the theorist

[Martin Czech]

:::Rules of thumb have their use, even though they can never be accurate.
:::The famous "one nH per mm of PCB trace" is useful to get a feeling for
:::the magnitude of inductance, but you can only know exactly when you also
:::know the return path (usually a ground plane).
:::
:::But before we start to frown about the rules of thumb, let's keep in mind
:::that
:::this whole concept of "inductance" is an approximation, closed loop or not.
:::Only valid in a quasi static case.
:::
:::I guess the only way to make a precise description of the "electronic word"
:::are Maxwell's equations - but gladly you can do without them in many
:::practical cases.
:::And when you cannot, sadly you always fight to get the right boundary
:::conditions ...
:::
:::In your example, try to solve for a finite wire length, and use a second
:::parallel
:::wire for a return path, and vary the distance to the first wire. Have not
:::(recently)
:::done it, but I think it might give you a feeling of what's going on.
:::Alternatively, use
:::an infinite conductive plane as return path, and also vary the distance.
:::

I aggree. But a wire WITHOUT ground plane or any other back path makes
still no sense.

m.c.