Re: [EE] measuring the magnetic field from a small trace carrying a < 50uA load?

Chuck Olson <[email protected]> writes:
> Martin -
> 
> You are correct, I should have said something about moving the head
> across the conductors, cassette tapes moved less than 2 inches per
> second so the head wouldn't have to be moved super fast ?.
> 
> The thing I've noticed with Hall effect devices was that they are
> really noisy, 50 uA probably not going to give much of a signal above
> the noise.
> 
> Chuck

	I figured it was something like that plus one would have
to have some sort of low-pass filter because 60 or 50 HZ magnetic
fields are going to be common due to all the transformers and
motors in our world.  

	I was once part of a group of people who dug a hole for
the base of an amateur radio tower and the neighborhood in
question had buried power and telephone lines.  A fellow from the
power company came out and had a portable detector which was
probably inductive but had a speaker so you could hear the hum of
the 60-HZ magnetic field around the underground power cable.  He
marked the ground across the yard, following the buzz so we knew
where not to dig.

	Too bad the telephone line which was also buried near
didn't radiate much because we only found that out when we came
across one heck of a tree root and, after the shovel cut it, the
man's wife who had been on the phone and wasn't too keen on this
whole deal came out to inform us we had cut their telephone wire.
That root had a lead shield and 4 black wires.  Ah, what they are
doing with recombinant DNA these days.

	We patched the cable  temporarily so the phone worked
again and, I guess, AT&T fixed it right a few days later.  I'd
much rather accidentally cut a phone wire than 200 amps at 240
volts phase to phase.  An analog telephone line pulls a few
milliamps DC when a conversation is going on plus a little more
if it rings but in the year 1975 when this all happened, there
wouldn't have been any ADSL carriers or anything much on that
line to radiate through the few feet of soil to make precise
location easy.  When a phone is on the hook with no conversation,
there is 0 DC current being drawn.  It would have taken a pretty
good metal detector to find that line.  I digress.

Martin