From: jlevine@rd.hydro.on.ca (Jody Levine)
Subject: Re: Courier bag:  Do you have one?
Organization: Ontario Hydro - Research Division
Distribution: na
Date: Mon, 25 Jan 93 16:09:40 GMT

In article <1993Jan22.212815.461@pages.com> bigdog!jon (Jon Wright) writes:
>
>I asked somebody at MotoPort about this and they said the ceramic magnets,  
>which were more powerful than the previous metalic ones, were also polarized or  
>uni-directional or something, which prevented screwing up your disks and cards.   
All together now, "There's no such thing as a magnetic monopole!" No? That
wasn't what they meant? What did they mean, then?

Take two fridge magnets, you know, the rubbery kind, and try to stick them 
together. They will hold together, but only if they're offset by so many
milimetres. If you try to move slide them against each other, they won't
let you, or they will but will resettle somewhere else, but not in between.
These magnets are made so that they alternate (north-south)-(south-north)-etc.
along their surface, so the dipole is not one surface north and the flip side
south, (magnet a) but a bunch of dipoles laid flat along the surface (magnet
b). 

Now imagine the magnetic field of magnet a, with one side north 
and the flip side south. Draw two poles on a piece of paper and draw lines
through the magnet from north to south. Turn each of the lines into a closed
path by drawing lines around the outside of the magnet putting arrows on
the lines to show which way you went. See? Outside the magnet, no matter 
how far away you are, there are lines going through you and you're sitting 
in a magnetic field. It get's weaker as you go away, but it's still there. 
Now, on the same piece of paper, draw another dipole, in line with the 
first, but pointing the other way (you now have four poles in a row). Draw 
the associated field lines. If you're close to either of the dipoles, you 
feel the effect of that one. If you're far away, the fields cancel out and 
you feel nothing. Ok, so you have to be a lot closer to a string of opposing, 
end to end dipoles to be in the field. Likely, all the manufacturers have to
do is put a layer of foam between the magnet and the bag interior, or laminate 
some non-magnetic plastic to the inside of the magnet and you're out of the 
danger zone. If they're doing anything fancier, I'd be interested to know.

When it's sitting on your tank, things get even better for your sensitinve
tankbag contents. Field lines are picky, and would much rather be in a 
nice, ferromagnetic metal than in air, bits of plastic, or dirty socks.
If you drew your dipole with half the lines on each side, lay the dipole
down on your gas tank and some of the lines will move to the tank side,
leaving less field in the bag. This is why your bag sticks to the tank, as
having the magnetic flux return through steel is a lower potential-energy
state than having it return through air.

Conclusion:

All flat magnets are made this may, becuase it's cheap. You have to be really
close to them for them to work. Is it enough to erase floppies & cards
anyway? Don't know. Try it. Put rainsuits in the bottom of your bag until
it doesn't erase your floppies anymore.

Someone please try this:

Stick your old floppy in the bag, then try to read the floppy. Put your
old floppy between your tank and the bag, and then try to read it. I'd be
interested to know if there's a difference.

I've        bike                      like       | Jody Levine  DoD #275 kV
     got a       you can        if you      -PF  | Jody.P.Levine@hydro.on.ca
                         ride it                 | Toronto, Ontario, Canada