G'morning David, 

 DW> Ceramic magnets are, yes.

Yup - went home and hit one hard with surprising results...

 DW> On the other hand, a friend of mine was once assembling a big
 DW> electromagnet. He neglected to tighten the bolts sufficiently, so when
 DW> he turned the thing on, one pole-piece crashed into the other. He
 DW> couldn't separate them by force, so he reversed the windings so the
 DW> pole-pieces repelled each other. When he energized the coils, one
 DW> pole-piece broke free, flew across the room, and smashed through the
 DW> wall into the next room. He retrieved it from there, undamaged.

Sounds like he had an impressive power supply;  there used to be a 
power station and a huge rotary AC-DC convertor to drive the trams 
installed down at Kings Wharf in Auckland City here.

Air NZ (then called TEAL) had their aeroengine workshops close at 
hand,  and checked for fatigue and flaws in key metal parts by 
dousing them in oil and passing enormous DC currents through them.  
Internal flaws generated visible patterns in the surface oil.

My engineer father did some consulting work for them and learnt 
about this application.  We'd brought a Riley Kestrel 12/4 with an 
ex-ERA 6 cylinder engine out from England that kept on chewing out 
its half-axles,  so he had a batch of 6 new ones made and we went 
down and tested them with TEAL's gear  -  the sudden life in the 
surface oil with the bang of the switchgear was all very mind- 
bending.

:-)
  
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