Owen wrote in news:vtKuo.256$tk4.180
@viwinnwfe02.internal.bigpond.com:

Well, I didn't get the maths right, there was a sign error in the formula
below. Here is what it should have read.

Suppose you had an air cored inductor, that when you measure the
inductance of the first half of the inductor (other terminal open) you
get 10µH. You now measure the whole inductor and get 30µH. We can
calculate that M=5µH.

Now forming a T equivalent of the inductor with one half shorted,
L=10-5+(5//(10-5))=7.5µH. Notably, the current in the s/c is
50% of the current in the other section, so losses are about 25%
of that in the other section... not usually a big issue.

Of course, the situation depends on the tapping point, and is much worse
when you short just one turn... but we don't usually do that. The example
has a fairly high coupling factor k (for an air cored coil), and losses
are lower for lower k.

I am working on a note that expands on this.

Owen