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

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.

-- Joe Legris

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.

--
Bill Sloman, Nijmegen (but in Melbourne at the moment).

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 =^.^=

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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" snipped-for-privacy@sjm.com wrote:>

** No they are not.

You need to take basic reading lessons - f****it.

...... 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

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.

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Thanks,
Fred.

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

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