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How to get/make 'non-magnetic bifilar heating wire'?

  1. Apr 27, 2014 #1
    I'm looking for heating wire (to ~200-300 degrees Celsius) that won't disturb a ~weakish applied magnetic field significantly - I would be wrapped it helically around a glass cell which is placed inside magnetic field coils. So if I had two parallel sets of non-magnetic wire attached and coiled together, carrying equal and opposite currents, I believe it would not affect the applied magnetic field.

    I have seen such a setup in Teachspin's Optical Pumping experiment, where the manual describes the wire as 'non-magnetic bifilar wound heater wire' which I am *guessing* is nichrome/kanthal (could be others though), but I can't find bifilar nichrome/kanthal heating wire for sale anywhere, and I'm not sure if there's a way to safely construct it by myself.

    Anyone have suggestions for where I could acquire it or something equivalent, or am I using the wrong terminology here?

    Is it possible to make it by myself out by somehow joining two sets of heating wires together?

    Any help or ideas at all would be greatly appreciated!
  2. jcsd
  3. Apr 27, 2014 #2


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    If you can twist the insulated heating wires to make a twisted pair, that will disturb the local B field the least. Is the heating wire insulated?
  4. Apr 27, 2014 #3
    That's the problem though, like I said I'm not very familiar with resistance wires but intuitively I think if the goal is for heating then I'd assume they wouldn't be insulated - so I'm not sure how twisting would work without insulation from a safety point of view if the high levels of contact might cause damage (though I could be completely off the mark on that).

    One company wrote back to me, they do sell bifilar nichrome but it is out of my price range (~1500 USD) - their sales member said that the wires are bonded with an adhesive, usually either Polyvinyl Butyral 105 C which is soluble in isopropyl alcohol or a non-alcohol soluble polymer, polyurethane (most go with the alcohol soluble type for convenience in separating the strands for termination).

    Does anyone have any experience in making it themselves using adhesive?
  5. Apr 29, 2014 #4
    Okay I have done a little more research on this - so now I understand that without insulation it won't work at all because they will short-circuit.

    BUT using a twisted pair is pretty easy because you can just coil them with a drill and then they will stay together like that - this minimises disturbance of the applied magnetic field but does come at the cost of thermal efficiency, since it would be lowering contact with the cell.

    However the thermal efficiency is already substantially lowered by the fact that the wires would need to be insulated from each other...

    So I guess the real aim is to find a non-magnetic resistance wire that has a thin layer of insulation which decent thermal conductivity, but one that will not melt/be damaged by the high heat - if I was aiming for 200 degrees on the cell that would likely need ~300 degrees in the wires.

    Does anyone have suggestions for material for either the resistance wire or the insulation? Is what I am searching for realistic, or overly optimistic?
  6. Apr 29, 2014 #5


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    What is the length of the bifilar wire you need?
    What is the maximum diameter of the twisted pair?
    How many watts will you be generating?
    What power supply voltage current is available?
  7. May 5, 2014 #6
    Your notion of having two wires with current running in opposite directions should be relatively effective, because they will tend to generate fields of opposite polarity. However, I don't see any need to get bifilar wire. If you can bend it, it should be just as effective to take your wire, fold it in the center, and wrap it around your glass cell. That way, the current will travel one way to the fold, and return in the opposite direction.

    Also, how closely together will the heating wire need to be wrapped? Can you use uninsulated wire and insert some sort of spacers (possibly hand made) between the filaments every so often, to keep them from shorting? If you're using the folded wire I mentioned, being shorted would not be a complete disaster - it would just shorten the amount of your cell that is receiving the heat.
  8. May 5, 2014 #7
    Well a lot of this is flexible - the cell is relatively small, a glass cylinder (length ~100mm, circular face diameter ~70mm). It would be filled with air of varying pressure and need to heat it to ~200 degrees max, I am unsure of how to do the calculation to figure out the Watts required (something to do with specific heat/amount of gas molecules? Note that time is not a major issue here, I don't mind needing to wait for the cell to heat). So I would probably need 2-5m of wire depending on how much spacing is in between coils.
    Right now I have two power supplies, 32V/10A and 60V/50A, I would ideally use the first one but the second can be used if required.

    That's an interesting idea, how do spacers work and how do I make them?

    The other alternative I have considered is to make a twisted pair out of one resistance wire and then some other insulated wire (copper?) and join them at one end - essentially creating the same single coil, but not risking shorting or requiring insulation on the resistance wire.

    Are there insulated resistance wires on the market that safely take 200 degrees? That is probably the easiest/most efficient setup.
  9. May 6, 2014 #8
    Heh. I really have to think things out more thoroughly before I suggest them to other people. I usually think of some kind of jerry-rigging for myself, then have to go figure out how to do it.

    I DID, however, come up with a way. Wrap the wires around your glass cell, then go over it carefully to make sure there are no places where the wire is shorted. Next, take some Engine Enamel (something like this: https://www.amazon.com/Glyptal-1201...&qid=1399384909&sr=1-8&keywords=engine+enamel - note: I am not affiliated, it's just what came up in a search) and paint some lines down the cell over the wires to keep them from being moved when the cell is being handled.
    Last edited by a moderator: May 6, 2017
  10. May 6, 2014 #9


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    The power required will be determined by heat loss, or how much insulation you do not place outside the cell and resistance wire. If there are optical apertures at the ends of the cell then there will be heat loss there.

    The best power source would be low voltage high current supply. That would permit you to use thicker wire.
  11. May 6, 2014 #10

    jim hardy

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    automotive stores have an interesting array of handy stuff.

    Look into silicone exhaust gasket sealer.
    Last edited by a moderator: May 6, 2017
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