On Oct 25, 2007, at 10:08 AM, Stephen A. Lawrence wrote:
William Beaty wrote:
On Thu, 25 Oct 2007, John Winterflood wrote:
thomas malloy wrote:
As Jed pointed out, a pair of heavy iron frying pans might make a
superb
Faraday cage.
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.
You can't have a time varying B field without an accompanying time-
varying E field; in free space (or, for the most part, air), Del X
E = -(1/c)dB/dt and Del X B = (1/c)dE/dt assure that.
So, if we can block the ELECTRIC component of a EM field, we must
be able to block the MAGNETIC component too. Conversely, if the
MAGNETIC component comes waltzing blithely through the shields, it
must surely bring along the ELECTRIC component as well -- or so it
would seem to me.
What am I missing?
Given 60 Hz is carried by wire that often encircles rooms, the 60 Hz
magnetic field can be largely a near field effect, not EM radiation.
Horace Heffner
http://www.mtaonline.net/~hheffner/
