Centre tapped coil with in a coil - inductance?

[peskywinnets]

What happens if you wind a coil around an iron core, stop, centre tap it, then wind the same amount of turns around (the first coi) again? (you end up with a coil within a coil)

I'm particularly interested in what the inductance would be for each half.

For example I know that..... 100 turns of 0.3mm wire yields about 0.8mH

so for this particular coil scenario, I end up with ....

100 turns of 0.3mm wire = 0.8mH

ground tap

100 turns of 0.3mm wire = 0.8mH

if putting an AC signal through one half of the coil will it affect the other half? (yes I know each leg will require an inverted signal...but I'm wondering if the AC signal through one coil impacts the other, therefore playing havoc with the inductance aspect)

[Bob Scott]

Yes. It becomes a poor transformer. Both sides are coupled to each other by the core.

The inductance of a coil is proportional to the square of the number of turns. However, the inductance fo two independent inductances in series is the sum of both.

The square formula only works if both windings are on the same core.