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Re: Magnets (making them)



---------- Forwarded message ----------
Date: Wed, 1 Oct 1997 08:50:11 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Magnets (making them)



Gary Weaver wrote:
> I have been checking catalogs and have found many companies that sell
> magnets.  Having access to a machine shop I would like to make my own
> magnets the size, shape and design I want.   I have been researching
magnets
> and they are made of iron, aluminum and cobalt.   I checked with a metal
> company but they can't tell me much.  Does anyone know of a metal that
can
> be used to make magnets.
> 
> I built a electro magnet with 2 coils 2000 turns each of #24 wire.  Power
> supply is 170 VDC 7 amps.  I can increase the power to 678 VDC 7 amps. I
> used it to increase the power of several old magnets.  I increased the
> magnetisum of 6 round magnets.  I had no luck and increasing the power of
3
> horse shoe magnets. If I could find the correct metal I could make my own
> magnets.
> 

There are some books on magnet design, which at the level we tend to work
at is a craft or art, rather than a science. If you happen to have access
to a good Finite Element Analysis program, you can make it more of a
science.  Anyway, there is a distinct difference between what makes a good
electromagnet core and what makes a good permanent magnet.

For an electromagnet, you want high permeability, but low coercivity, i.e.
soft iron.  The magnetic field is a linear function of the number of turns
and the current through the turns up to a limiting value called saturation,
above which the permeability appears to decrease.  The only way to get more
field is to increase the cross section of the core.

For a permanent magnet, you want something that once magnetized, stays that
way.  Alnico is an example of a nickel alloy that has this property.  you
make your shape out of unmagnetized Alnico, put it inside a coil, run
current through the coil sufficient to exceed the coercivity, and when you
turn off the current, you've got a magnet. In industrial applications, the
current is applied by a capacitor discharge circuit since it is only the
peak current that is important, not the duration.

It turns out that certain rare earth ceramics containing elements like
samarium, have extremely high coercivities, which makes for a lot of field
in a small package. This is good for small powerful electric motors or
headphones, etc. As I understand it, the manufacturing process, though, is
somewhat more complex than for the straight metallic alloys.

Why not just call a magnet manufacturer? They aren't particularly expensive.