In a message dated 03/09/2000 16:05:41 GMT Daylight Time, gbl@....... writes:

> It has been mentioned that iron makes a better material for sensor
> feedback coil transducers than steel. 

    It does. Silicon iron may be even better. This is one of the more common 
materials for making transformer laminations. Permalloy laminations are used 
in some 'microphone' transformers.

> I have two questions which maybe someone can help. Do manufactured speaker
> coils have a better grade of material for use in feedback coils? I have
> looked around a little but can't seem to find a source of iron plate or
> round stock. Any thoughts?

    Speaker coils themselves are made from Cu or Al wire and are sometimes 
wound on a sheet of Cu or Al to give good heat transfer. If you want to make 
LVDT sensors, you can roll a strip of transformer lamination into a tube. 
This will allow you to use a higher frequency excitation since you are then 
only magnetising a thin tube. Commercial sensors use one of the various 
Permalloys to give a high permeablilty and low remanence. 

    I believe that the reinforcing rod used for ferro concrete is almost pure 
iron and is quite soft, both physically and magnetically and is quite OK for 
making pole pieces etc. 'Mild' steel is also OK for pole pieces where you 
don't need low remanence. I hope that this helps. It was not very clear from 
your letter what you were thinking of making.

    Regards,

    Chris Chapman
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