On Thu, 25 Oct 2007, Stephen A. Lawrence wrote: > William Beaty wrote: > > Yes, and they solve the problem of shielding low-freq magnetism. For > > example, to well shield the magnetic component of 60Hz you'd need many > > inches thick of copper. > > I've heard this statement, or others very much like it, a number of > times and I still don't understand it.
Experimentally, you can shield a 60Hz voltage field with aluminum foil, or even with a micro-thin metal coating on glass. It's very easy to "short out" the e-field. But the b-field from a 60Hz coil goes right through aluminum foil as if it wasn't even there. A quarter inch of copper will cut down 60Hz magnetic signals, but to actually shield them you need way more than 1/4 inch. It's the difference between coils and capacitors. > So, if we can block the ELECTRIC component of a EM field, we must be > able to block the MAGNETIC component too. A thin aluminum box will block all voltage fields down to DC. So why shouldn't it also block all magnetic fields down to DC? Yet it doesn't. And a box made of nonconductive ferrite slabs can block the magnetic fields while letting the voltage fields pass right inside. (((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website billb at amasci com http://amasci.com EE/programmer/sci-exhibits amateur science, hobby projects, sci fair Seattle, WA 425-222-5066 unusual phenomena, tesla coils, weird sci
