We too have suffered from this problem in the past. Our solution is to
keep a piece of blue tack handy and to dab it onto the bottom of the magnet
in the places where the metal collects on a regular basis. The minute
traces of metal stick into the blue tack which can then be folded over to
trap them for ever.
Incidentally, I suspect that it is not just iron filings that cause this
problem. The traces we find are generally extremely small and may have
come from machining of metal anywhere, but most likely from our workshop.
Any small piece of metal will be attracted to a powerful magnet provided it
has a residual miniscule ability to be magnetised. Anyone going into the
workshop and then walking into the NMR lab could introduce these traces.
Blue tack is the name given in the UK to the plasticky sticky stuff that
you can use to stick notices to notice boards.
Frank
Frank Riddell, School of Chemistry, University of St Andrews,
St Andrews, Fife, KY16 9ST, Scotland, UK.
Tel: + 44 (0) 1334 - 463815 Fax: + 44 (0) 1334 - 463808
E-mail: fgr@st-andrews.ac.uk
Research group page:
WWW: http://www.st-and.ac.uk/~www_sc/personal/fgr
Personal Photos page:
WWW: http://www.st-and.ac.uk/~www_sc/personal/fgr/Photos/