Measuring Current in a Magnetic Field: A Physics Problem...?

Hi Everyone,

I have a pretty difficult problem that I would really appreciate some
help in solving.

I need to measure the current induced in a conductor by a strong
magnetic field. I need to do this without using any conductive
material in the magnetic field since this will induce current in the
measurement equipment and trash the results.

Here are ideas that I've been tossing around but do not have a real
grasp on.

Can I use some light emitting diode and pump the light out in a fiber
optic cable and sample it away from the experimental setup?

Can I submerge the conductor in a liquid, measure heat generated by the
current?

I'm at a loss on how to approach this problem, any advice?

Thanks!

 

There's no such thing as a perfect measurement - if you extract
information from a physical process you change it a little. There's no
getting past it. Your job is to figure out how much error is tolerable
and what you can do to minimize and/or correct for it.

Tell us more about your problem. What is the environment? Why are you
doing this? Where is the magnetic field coming from? What and why is
the conductor in there. Why do you need to measure the current? To what
precision? Roughly how big is each? Give us some numbers.

 

1) Your conditions are contradictory; "need to measure the current
induced in a conductor" does not fit "without using any conductive
material".
Make up your mind.
2) The magnetic field mentioned, "will induce current in the measurement
equipment" if there is any relative movement.
3) Better make a more accurate descriptionas to what you would like to
measure *first*; perhaps some practical ideas may come to mind.

 

Of course, one can ask *what* current if no conductor is desired...

 

** Groper Alert !

** That an alternating magnetic field ??

** Where ever did you get that mad idea ??

Co-axial and twisted pair cables are virtually immune to induction from
magnetic fields. They have been working to keep microphones free from AC hum
and antenna cables free from interference for the last 70 years.

So should even work for you !!

** Your grasp seems strong on just one small thing.

** No way to help with a specific solution with so little info from you on
the actual problem.

Get real - anytime.




........ Phil

 

Excellent question; i had ASS-u-ME-ed that it was a DC type.

 

If you have a current-carring conductor in a magnetic field it is
experiencing a force or a torque, depending on the geometry. Find a
couple of spring balances (with glass-fibre springs) and measure the
force.

 

Or you could use a low voltage filament bulb,
but this or the led would cuase an increase in resistance wich will lower
the current,
if you want to measure it more accuratly you need to amplify it,
a very small amplifier in the magnetic field might be able to be sheilded
enough,
or with carefull layout magnetic effects could be canceled out,
if the signal is amplified suficiently twisted wires from the amplifier
would have little error,
or digital pwm could be used wich is imune to induced voltages,
or via optical cable.

If however the effect of other conductors in the field skews your result
though then I dont know what the alternative is.

Is it possible for any part of the conductor to leave the magnetic field in
a loop ?

Colin =^.^=

 

These requirements are mutually exclusive. You need to go back to your
teacher and have her or him clarify this assignment, or maybe take
the prerequisites again.

Good Luck!
Rich

 

"Richard The Dreaded Libertarian"

** No they are not.

You need to take basic reading lessons - fuckwit.




....... Phil

 

OK, I guess I need to clarify my problem.

And by the way, this is not a class assignment as some of you have
implied. This is for a very serious application. But it so happens
that I am not an electrical engineer, and so i may have phrased my
question in a manner that was peculiar to those skilled in the art.

I have a conductive, metallic device (see geometry below) that is
submerged in a liquid solution. This entire setup is subjected to a
high intensity
varying magnetic field (dB/dt).

What I need to measure is the intensity of currents induced into this
conductive device. For example, imagine my device looks like this:

**********
***********************************************************B
A *******
***********************************************************C
**********

I want to measure if any currents are flowing from A-C, A-B and B-C.
And I also need to estimate the intensity of those currents. I need
to measure currents as low as 1 milliAmp.

My problem. If I hook this up to standard measuring instruments, the
strong magnetic field will/may induce currents in the actual measuring
apparatus and thus my reading would be completely wrong. I'm at a
loss as to how I can achieve this measurement. Fiber optics method?
anything?

Thanks everyone, (at least those who actually tried to help me, to
Richard The Dreaded Libertarian, you assume too much, not a sign of
high IQ).

 

You could treat the conductors as antennas.

If the conductive elements are different lengths, then you might be
able to measure the absorption at different frequencies. It depends if
the elements function together as one array of if they are isolated.

A conductive element will absorb current at multiples of 1/2 the
electrical wavelength, which is altered by other conductors,
dielectrics (and liquids) in proximity to the conductor.

Does your application require knowing the current at resonance or at
some specific frequency?

Frank Raffaeli
http://www.aomwireless.com/

 

Frank Raffaeli a écrit :

First it would be helpful to know the mag field frequency, and size
(length, spacing)of the conductors.

Second, varying mag fields don't induce currents into wires but voltage.
If you don't close the loop (and if frequency is small enough so that
wavelength is long WRT to system dimensions) there is (almost) no currents.

Third, it is easy to measure mag fields without disturbing it too much
(a small loop at the end of a small coax cable), then from the measured
field deduce the induced voltage. Well your system seems to be simple
enough to allow this.

But first you'll have to disclose some real figures.

 

There are some approximations you should consider, first:
is the skin depth of your conductors large compared to the
size? Skin depth refers to the ability of conductors to
shield their interior from external applied magnetic fields,
it's well studied and lots of info is available. Start by
looking up the skin depth for your material and frequency
of interest.

Skin depth >>size means the magnetic field inside the conductor
is nearly the same as if the conductor wasn't present.

Skin depth << size means the surface of the conductor holds all
the currents that are induced (this limits the geometric scope
of the currents to the surface layer).

Another idea is to remove the conductors, measure the B field,
then reinsert the conductors and remeasure the B field; the difference
is due to currents in the conductor parts. This assumes that
you can map a significant volume of the field to good spatial
resolution. And that you can invert the field to make a model
of the currents.

At the most-difficult end, this problem is one of antenna theory,
and it isn't usual to completely solve such problems (though
the behavior can be characterized in lots of simple ways,
the total solution of all currents isn't often attempted).

 

Yeah, thanks for noticing. ;-)

Why not just put a shunt from A-B, one from B-C, and one from A-C,
and run twisted pairs to some voltmeters?

And we do get an awful lot of homework questions, mostly underspecified.

Good Luck!
Rich