# Magnetic field immune wire?

1. Apr 4, 2012

### Patokun

Hello all,

Are there any cables that are immune and not affected by magnetic fields of the strength of 1-2 Tesla? Either simple current wires, or more complex ones like USB or Ethernet cables.

2. Apr 4, 2012

### M Quack

You mean cables that don't pick up electromagnetic noise or induced voltages when you ramp the field?

A static field should not have much effect anyways.

Coaxial or triaxial (ground shield around signal- and signal+) are best, second best are twisted pairs, shielded if necessary.

3. Apr 4, 2012

### Patokun

I don't think its static. It's a field similar to ones that are found in IRM machines. (As far as I know those are electromagnetic)

Don't coaxial cables transmit data using magnetic fields? I thought that would make them more vulnerable to an electromagnetic influence. (Then again, I don't know much about that)

I knew twisted pairs are good at resisting magnetic influence, but I was wondering if they can do that with fields as strong as 1-2 Tesla?

Thank you for the help!

4. Apr 4, 2012

### yungman

I don't think you induce EMF unless you have the cable formingf a loop as per Faraday's law.
Also it depend on the frequency of the magnetic field ( which is EM when it is varying as E and B coupled by Maxwell's equation). For high frequency, people put a toroid on the cable to break the loop.

5. Apr 4, 2012

### jim hardy

the idea is to keep the area enclosed by the wires as small as possible.

A tesla is a weber per square meter and if you keep the square meters miniscule there's not a lot of milliwebers coupling the loop. Twisting helps the induction in each segment cancel that in the preceding one.

That's the very basics in simple terms. Yungman is expert.

How are you getting that sort of field in air?

What is this IRM, some kind of rolling machine ?

6. Apr 4, 2012

### M Quack

At 1-2T this is bound to be superconducting magnets (or permanent over a very very small volume). The main field will vary only very slowly, on a time scale of minutes. The induced voltages will be minimal.

However, you should make sure that your cables do not move in the field, as it will have a strong gradient.

At very high frequencies (MHz) yes. But you can perfectly well use coax cables for low frequencies and DC, then the magnetic part can be neglected.

In any case, even in a static (or slowly varying) background field you can still use coax cables to transmit high frequency signals.

Yes. The rate of change is more important than the absolute field strength. I know people that use twisted pairs (and also coaxial cables) in pulsed magnetic fields of 60T and more. Make sure the twist is nice and tight, with as little space between the wires as possible, and that the pitch is regular.

Note also that your magnet will have 1-2 T in the field center. The further you go away from that, the lower the stray field will be. IMR machines sometimes are compensated such that the stray field is very small.

NP

7. Apr 4, 2012

### davenn

The best magnetic/electric field immune cable I know of is optical fibre

Dave

8. Apr 4, 2012

### yungman

:rofl::rofl::surprised

9. Apr 5, 2012

### Patokun

No it is not. We're simply doing some wiring for some equipment in the IRM room, and although the equipment is sealed inside magnetic insulation boxes, the wiring is not yet done.

I have to admit I am feeling out of my league here @_@

I've got no idea how the IRM field works, but I'm going to trust you on that one. 60T, wow >_>

I can't test directly on the IRM for now, but I got a loudspeaker magnet that looks like this one, which I heard has around 1-2T magnetic field, but I have no idea if the field it makes is similar to the one found in an IRM. I know it wrecked havoc on an old CRT screen I have.

Do twisted pairs have to be those of Ethernet, do I have to use an Ethernet card to use it? Or can I transmit data on the wires like they were normal copper wires? Is it for the same for coaxial cables? If yes, then my problem will be solved! I'll just buy cat6 cable and strip the ends and use that.

Yeah but the encoders/decoders of those cost a fortune ~.~

For such a distance and the really low bandwidth we have it's not really worth it.

And thank you all for the replies!

10. Apr 5, 2012

### M Quack

For computing, you will be fine with standard, off-the-shelf cabling. Mains are no problem at all.

For sensitive instrumentation, try to go with coax or twisted pairs, and above all avoid ground loops.

Everything else is probably overkill for your case.

11. Apr 5, 2012

### yungman

Get any cheap two wires pair from the store, then pull out the length you need. Make sure you have extra length, secure one end. Then use an electric drill and fix the other end on the chuck and turn the drill on to twist the pair. Make sure you pull tight to prevent from tangling. We did that all the time to make twisted pairs.

Last edited: Apr 5, 2012
12. Apr 5, 2012

### skeptic2

Magnetic fields pass easily through nonferrous metals. Using twisted pair will help but I don't think shielded cable will do much. Have you considered conduit made out of a ferrous metal.