In a message dated 13/06/00 00:54:31 GMT Daylight Time, CapAAVSO@....... 
writes:

>    I want to cut and bend some mu-metal to make a core for a flux gate. I 
>  have been told the core will need to be annealed to restore its 
permeability 
>  or the flux gate won't work very well. Can anyone tell me how to anneal a 
>  toroidal core about 5 cm in diameter?

    When we had some tubular Mu-Metal shields made to screen 
photo-multipliers, they were rolled from sheet, Argon arc welded and then 
heat treated in hydrogen. If I remember correctly, the hydrogen prevents 
oxidation and also reduces selective evaporation of the alloy. I don't 
remember the temperature. The heat treatment anneals the metal and allows 
crystal growth. The final product was definitely rather brittle.
    The straight fluxgate sensors used to be made from insulated wire to 
reduce eddy current problems. If you have really thin foil, you could 
probably just roll up a multi turn coil, but the metal probably wouldn't have 
it's optimal magnetic properties. This might not be too critical in a 
toroidal core so long as the whole length had the same properties. If you can 
get a used tubular welded shield, could you varnish it thoroughly, scribe it 
and then etch rings from it? Dud Geophones? CRT shields? (Mine cost 60 c)
    There are some 'new' ferrites on the market with 'square' 
characteristics. See FairRite, Arnolds, muRata, Ferroperm, Siemens (and 
Philips if you can get their * Website to work) Ferrites tend not to have 
serious eddy current problems, but may be more temperature sensitive. METGLAS 
alloys are also available with phenomenal properties. See Honeywell at 
http://www.electronicmaterials.com/businesses/sem/amorph/page5_1_2.htm

    Sorry not to be more encouraging. Hope that this is of some help. It 
rather depends on your application. I would just try making a core. You would 
probably need more drive field, but so what? 5 cm sounds like a Jumbo size. 
Several companies market fluxgate sensors, but that's not like making your 
own.

    Regards,

    Chris Chapman

    
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