In a message dated 18/11/2000, gbl@....... writes:

>  would the use of ferrite over
>  steel improve the magnet field concentration and therefore sensor
>  performance. I have seen various ferrite shapes. I'm not sure how well
>  it machines

Hi there Barry,

    The flux density of ferrites is lower than Alnico or Ticonal so you need 
mre of it, the temperature coefficient is much higher and it can only be 
machined by wet grinding. If you don't demagnetise it first, all the ground 
material sticks to the magnet. Then you have to magnetise it again. It is 
often used as large chunky disks in cheaper loudspeakers, but they may be 
glued in place, which makes the soft iron pole pieces difficult to machine 
(if you need to increase the gap). With the normal speaker constructions, you 
get a lot of stray magnetic field. You may wish to use it for cost reasons, 
but other permanent magnet materials are likely to give you 'better' 
performance with smaller magnets. I would not choose to use magnetised high 
carbon steel either. Mild steel is OK for pole pieces. It is better if it is 
either electroplated or painted to prevent corrosion.

    Regards,
    Chris Chapman
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