On Thursday, December 10, 2015 at 1:00:55 PM UTC-8, Ralph Mowery wrote:
wrote in message

At college I had a lab ... There were boxes of parts that
were suspose to be the same parts. Some of the parts were either bad or out
of spec. Not on purpose, they just got that way over the years. Me and a
person I was with usually could locate the bad parts and get our project
going first. Got to be a joke that the ones that got theirs to work had the
lucky box with all good parts for that design.

I was teaching one such lab, with bar magnet/coil experiments, and had
an inspiration. I got some iron filings and sheets of paper, and
had the students lay the paper over their bar magnet and sprinkle
the filings over it.

There were a dozen bar magnets in the 'materials' box, and half of 'em had odd
fields. One had five identifiable poles. Using only the dipole-type
bar magnets, the class got better compliance than usual with the
expected behavior of poles and coils in motion.