On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

The characteristic that controls this is the
product of the magnet length and the material Coercive Force
- roughly speaking this controls the ability of the material
to retain much of it's magnetic strength during or after
exposure to an airgap in the magnetic circuit.

A magnet is open circuit if it's on the bench with
no pole pieces attached - the magnetic circuit is closed if
a continuous path through soft iron connects North pole to
South pole.

All the older types of magnet steels and even the
latest Alnicos behave this way and 50 to 90% loss of field
strength from incautious disassembly is possible.

Ceramic (ferrite) and rare earth (samarium etc)
magnets behave differently. Their coercive force is so high
that they can withstand being magnetically open circuited
without serious loss of field strength.

Jim