On 01/01/2010 12:59 PM, William Beaty wrote: > On Fri, 1 Jan 2010, Stephen A. Lawrence wrote:
>> An ideal toroidal coil has no external field -- symmetry and simple >> arguments regarding the curl of the B field show that it's got to >> be null outside the torus. In particular, any loop around the >> outside of the torus must have zero >> net B field (if we integrate it around the loop) > > Yep, that's exactly it. > After a little more thought I realized I have no idea what the Steorn toroidal magnetic cores have for a B field. Anybody got a link to a picture, or could someone who knows how the field is shaped sketch it? B field lines are always loops but the field loops from a permanent magnet *must* intersect the material of the magnet (or be knotted around the magnet like the loops around a solenoid), so the field of a toroidal magnet can't be doughnut rings around the outside of the torus, as one might tend to imagine it. So, what *is* the shape of the field? And how can a toroidal coil wrapped around the core, which necessarily has a very different field shape (lines not even close to parallel to the field lines of the magnetic core), quench the field of the core? My presumption is that the effect of the coil's field on the core is to rotate its field so that it's parallel to the coil's field, at which point the field of the magnet, like the field of the coil, becomes invisible outside the torus. But that's really just a wild guess, based on bits and pieces of what Bill Beaty has said about toroidal core saturation.
