Chris Thomas said:
Good grief; James Maxwell is rolling over in his grave. You might
want to read a freshman physics textbook. The field of ANY magnet,
whether it's an electromagnet, cheap iron magnet, or one of your rare
earth magnets varies with distance in exactly the same manner. If
you can demonstrate differently, I suggest you contact the Nobel
prize committee in Stockholm as you are a candidate.
You're missing my point. If you had a normal magnet of equal strength as a
R.E. magnet, it would be physically much larger - and the field would extend
to a greater distance than the R.E. magnet of equivalent strength. Hence
these magnets use in things like hard-drives where an ordinary magnet of
equivalent strength would play havoc with the data.
"The size of a magnet has a lot to do with the perceived strength of its
field, though. None of these magnets are very big, so that inverse-cube-law
field strength reduction bites into their power quite quickly.
Chisel the huge ferrite disc magnet off the back of a large dead speaker (if
it wasn't dead before you started chiselling, it sure will be when you've
finished) and you'll have a magnet with only about 1000G field strength,
measured at the peak strength areas on its poles. It's a ferrite. That's all
you get.
But big speaker magnets commonly weigh more than a kilogram and are several
inches across. The peak strength areas at the poles are thus already a few
inches away from the middle of the magnet's field. In this case, you can
move another few inches away and still have 1/8th field strength.
So if you wave one of these big magnets over a pile of nails, they'll leap
up to stick to it from several inches away.
Take a 1-Tesla-field-strength neodymium magnet the size of a button, though,
and the peak field areas on the outside of the magnet will only be a couple
of millimetres away from the middle of the field. Now moving just another
couple of millimetres away gives you 1/8th field strength. Field close to
magnet stronger; field far from magnet weaker."
