Horace Heffner wrote:
On Oct 24, 2007, at 10:54 PM, John Winterflood wrote:
The important thing about a Faraday cage is that inside it you cannot
tell anything about electric fields or electric potentials that exist
outside. You can't tell (in theory at least) whether the cage you
are in is grounded, or sitting at 100kV, or on the top of a Tesla
coil and being oscillated plus and minus to many megavolts.
In this Ron's case however there is an "ground" wire entering the
cage and who knows what potential difference exists between the cage
and the wire entering it until he measures it. This is the important
thing - it doesn't matter whether either or neither are grounded - it
just matters what is the AC and DC difference in potential between
the wire entering and a well constructed cage.
Good point. Another option along the same lines might be to simply
strip a section of the ground wire and connect the ground wire to the
faraday cage at the entry point using an alligator clip. It the
lights go out then the power is from an external source.
Wow, that's perfect!
Now why wasn't this obvious to start with? Dunno -- maybe it was to
others, but it sure wasn't obvious to me, at least.
To paraphrase: Local "ground", for objects inside the cage, is the
_inside_ _surface_ _of_ _the_ _cage_. So, a "ground wire" attached to
the circuit must go to the inside surface of the cage. A wire leading
out of the cage to some other potential is not a "ground wire", in any
real sense, for objects inside the cage.
Horace Heffner
http://www.mtaonline.net/~hheffner/
