What does a magnetic field strength of 0.418A/metre at a distance of 4.6 metres mean in real terms?

I'm not even sure if there's an answer to this question but thought I'd have a go - this is the postal restriction on sending magnets, and as I'm about to try to rehome a hefty pair of loudspeaker drive units (10" / 150W) I'm wondering if they're likely to be way above, way below, or somewhere around this, I have absolutely no concept of how strong a field "0.418A/metre at a distance of 4.6 metres" is! 

Anyone got any thoughts?



  • Hi Andy, check out magnet fishing on you tube or similar, speakers have been shipped for many years no problems, but I have wondered how strong magnets get through the postal system ok

  • It is a rather odd value, and leads one to wonder how the post office derived it.  Id be far more worried about near in fields and things sticking to the machines.

    The first thing is to convert to Teslas or Gauss, but being educated SI I will use Teslas in prefernce to Gauss, in the air around the magnet, the H and B will be parallel and in the ratio of  u0 -1.27E-6

    so  1.27 micro Tesla per amp/metre,  (The unit is equivalent to inductance of free space in uH per metre or micro newtons per square amp depending ehat you want to calculate, but here the Tesla per amp per metre is fine.)

    So 0.4A/m is approx  0.6 microtesla.

    This figure  is just crackers as it cannot easily be measured, as in fact it is far is lower than the field of the earth itself - that being around the 30-60 microtesla mark, depending how close the poles (higher) or equator (lower) you are.

    Now the field in the gap between the poles of a saturated hard steel steel magnet of the kind we grew up with, is about 0.8 to 1,0 tesla, and in a modern rare earth magnet, a touch higher but not by a factor of more than 2. In the pole gap of your speaker this will be the sort of exposed field level.

    So your question is how big a pole gap can be exposed for a saturated magnet, that will have fallen away to below 1 part in a million by a distance of 4.6m.

    Here we need to recall that magnets are dipoles, and the field from a simple dipole falls as cube law of the ratio of the distance to the pole gap.

    So to with a geometric factor K (K = pi, pi/2 or similar depending on detail of magnet shape) if the pole gap is say 1mm, then by 1m away, the field is down by 1000^3, or 10^9

    Similarly by 4m away, the field from a 4mm pole gap is down by the same amount.

    I suspect that any magnet with an open pole gap of less than about 10mm will be fine, regardless of geometric details.
    Anything stronger should be shipped with a keeper.

    It is a very silly threshold.



  • Says "5.3.26 Magnetised material with a magnetic field strength of 0.159A/metre or more at a distance of 2.1 metres from the outside of the package e.g. PA speakers."

    This www.supermagnete.de/.../Can-I-send-magnets-by-mail suggests it can be done in Germany at least. And suggests you test the package with paperclips 

  • Indeed and a paperclip test which will detect perhaps the low tens of milli-Tesla on the package surface, is a far more useful, realistic and easily performed and understood test.

    Well, unless you intend to add 2m of packing all round, which will  of course have the side effect  of  making it hard to get into the average letter box.


  • Speakers have holes in their frames to attach to the cabinets. Thinking you put threaded bar through these holes with nuts on the ends and have the speakers back to back. Tightening the nuts brings the magnets together and increases the repulsive force. So the field strength is used up in trying to push the magnets apart and might not extend much past vicinity of the package.

    Might work.

  • Many thanks all - on this last point actually I plan to strap them face-to-face to reduce the risk that the cones get damaged or misaligned.  In the end, if you buy a compete speaker unit (and I've bought far too many guitar / bass amps over the years!) they seem to get shipped with no problem. I do like the practical idea of boxing them up and then seeing if a British Standard paperclip will be attracted to it...

    P.S. I did actually look up to see if there's a BS for paperclips, I'm amazed to find there isn't! There's usually one for everything... 

  • Let us know how you get on Andy. 

    P.S. I did actually look up to see if there's a BS for paperclips, I'm amazed to find there isn't! There's usually one for everything..

    Quick, it looks like you've found a gap in the market! Smiley Submit your proposal.

  • oh no - please don't  - then we will have to fork out £100 or more for some hundred page tome  to read the tests needed to  see if our existing paperclips comply or not, in 5 years time only standards compliant paper clips with the right markings will be permitted,  and someone will be threatened with court action for using an old one because it could lead to a fatal accident. (I can't see how, but I'm sure someone will) We have more than enough standards as it is. Currently I have paper clips ranging from 12mm to 150mm between ends in various wire gauges in the desk beside me, and they all work just fine without any obvious signs of needing any sort of harmonisation or testing.