On a mailing list that I am on someone described an experiment in which they made a transformer where the primary consisted of a normal winding of insulted copper wire. The transformer was one designed to be easily disassembled for demonstrations.
They made a secondary of 15 turns of 1mm PVC insulated wire and measured the output, both open, and under load. Then they made an identical secondary except that it was encased in a ring of steel pipe with the ends not quite touching.
When the primary was powered up, the output voltage induced in the 15 turn secondary was the same whether it was unshielded or inside the ring of steel pipe. Open circuit voltage or voltage measured with a lamp load were not affected by the presence of the pipe.
The experimenters question was, if voltage induced in the secondary involves the coil cutting the flux as they referred to it, why was voltage not reduced when shielded inside the steel pipe. And a secondary question was, in a transformer design where flux is designed to stay in the core, how does it cut the coil and induce a voltage. Any flux which does escape the core is considered “leakage flux” and generally transformer cores are designed to minimize that. I’d add to that, a toroidal transformer where there is essentially no leakage flux is as efficient as you can get.
The whole cutting lines of flux concept is one I’ve never been comfortable with, I think it derives from the old iron filings experiments where they filings lined themselves up in lines that people assumed to be magnetic lines of force, but I think that assumption was incorrect; without the filings there is just a magnetic field gradient, not “lines”, the lines form because when one filing aligns with the gradient, it essentially short-circuits that magnetic field gradient over it’s length and so the magnetic gradient potential is higher near it’s ends attracting other filings, they in turn do the same thing and so the end result is filings self-organizing themselves into lines.
The questions are of interest. Working in various fields involving transmission of audio signals in shielded coaxial cables, I’ve always noted the lack of 100% effective shielding and attributed that to the fact that the shielding materials had electrical resistance and where therefore unable to completely exclude magnetic fields. I wouldn’t have been surprised that some field is induced in the secondary, after all the shield was incomplete, there was a gap, and steel is not a particularly good conductor and therefore shield.
That no difference in output was observed, that is surprising, and then the whole question about transformer theory and flux being contained within the core is also interesting. Please feel free to use comments or e-mail to add your thoughts on this subject.