NO copper will NOT, reduce any magnetic field you might have a problem with.
Please read these threads on the subject:
http://forums.procooling.com/vbb/sho...?threadid=2632
http://forums.procooling.com/vbb/sho...p?threadid=632
Or take the crash course presented here:
Iron or nickel will reduce your magnetic field.
All other metals are close to worthless in this aspect.
(there exists some special alloys of course but they are not for the average watercooler).