Is that right? We check for coil resistance. If the resistance of a coil changes (reduces), doesn't that mean that the power of the coil is changing?

Thanks JR (and Schudel5) - it's starting to make sense to me now. I've read that removing some windings from a coil makes it stronger, which would be the same as a partial internal short circuit? My problem is that this seems counter-intuitive, since removing (say) all but one winding would make the coil very strong? I ought to get this, as I first made an electromagnet in 1963! (Never too late to learn......)

Thanks JR. I don't want to hijack this thread, so I'll start my questions elsewhere when I've done some homework. I'm still confused because of this (typical) explanation:

"The magnetic fields generated by the separate turns of wire all pass through the center of the coil and add (superpose) to produce a strong field there. The more turns of wire, the stronger the field produced." (Wikipedia, electromagnetic coils).

OK - so now I've stopped scratching my head. Which was starting to hurt, after a month. Great, clear explanation. Thank you! So ... presumably there's a point at which removing turns will *eventually* make the coil weaker? Which means that if there is a major short in the coil (say, several layers of windings shorted through), it could make the coil weaker than normal?

Brilliant explanation, thanks!