I offered the info below to John Carmichael, a long-time friend. John
suggested I also copy the sundial reflector so here is what I sent to
John earlier.
John, I've followed the sundial list thread on using a magnet to secure
a gnomon. Let me provide you some information about rare earth magnets.
The most common magnet material is basically an iron/nickel alloy called
Alnico; it is not a rare earth magnet but just about every common magnet
you see is some form of Alnico.
There are two common types of rare earth magnet material: neodymium iron
boron (commonly referred to as just "neodymium") and samarium cobalt.
Samarium cobalt magnets can be more powerful (produce greater magnetic
flux) and can operate at higher temperature. Samarium cobalt magnets
typically cost more. Curiously, the expensive (because it is mined in
only a few places in the world) element is the cobalt, not the samarium!
Both types are typically plated with nickel and therefore look shiny.
Both are quite brittle and don't survive mechanical shocks well. Even
with the nickel plating, both types corrode when exposed to water vapor
for extended periods. The adhesion of the nickel plating is not great
and it is common to find it flaking off.
The mutual attraction of either type is very strong and you can get
yourself hurt if two magnets jump together and part of you is in
between! Both types are also strongly attracted to steel and some
stainless steels.
The air (or, glass or plastic) gap between mutually attracted magnets
strongly affects the attraction force. But, with either of the two
common types of rare earth magnet you would not want direct contact as
the separation force would be very high; I can easily imagine glass
breakage before separating the magnets.
Of course, the size and thickness of the rare earth magnet affects its
field strength and attraction force so you could experiment with
different sizes to select a pair with reasonable retention force.
Where the poles are located on any magnet is determined by how the
magnet was "charged" or initially magnetized. The raw material is not
itself magnetic, it acquires residual magnetism by being subjected to a
large magnetic field, usually in the form of a brief pulse from an
electromagnet. But, location of the poles matters when you want mutual
attraction between two magnets; magnets will self-align because opposite
poles attract. So, you need to know where the poles are located to
predict how the magnets will align to each other. A cylindrical magnet
can have its poles on opposite faces or at 180° opposing *edges* rather
than faces. Many magnets are made into special shapes to function with a
moving armature to produce a motor or actuator.
I can imagine that a small steel strip embedded in a stained glass panel
would adequately attract a rare earth magnet without the problem of
using two rare earth magnets in mutual attraction. But, you must select
the magnet size so that the force to separate the magnet from the steel
is not too large, or you will risk glass breakage.
You can go to eBay and search for neodymium magnets and you will find a
huge variety available. Buy some and play around.
Strong magnets must be shipped in a steel container to contain the
magnetic field. The post office gets quite upset if a package affects
their equipment!
Try dropping a cylindrical or spherical rare earth magnet down a copper
tube. You'll be surprised at what happens, noting that copper is not
ferromagnetic and is not at all attracted by the magnet. I have a
physics demonstration apparatus I made using a 2-inch wide, 1/4-inch
thick pure copper bar. I can roll a steel ball bearing down the strip
without impediment; but a spherical rare earth magnet rolls down very
slowly. Since the nickel plated rare earth magnet looks exactly like the
ball bearing, some slight of hand makes this into an interesting
demonstration piece.
I offer this little essay just for its educational value!
--
Best wishes,
Larry McDavid W6FUB
NASS Registrar
Anaheim, CA (20 miles southeast of Los Angeles, near Disneyland)
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