Sorry about a typing error. The Alnico coefficients are both negative, 
not positive.

>    Alnico has the lowest temperature coefficient of commercial magnetic 
>   material. However there are various 'grades' of  Alnico. Alnico 5 at has 
a 
>   temperature coefficient - 0.023%. Alnico 8H has a temperature coefficient 
of 
>   - 0.012%, but it may only be available in in large quantity.
>   
>       I haven't yet got a figure for Ticonal. Anyone ..?

> The spring constant of the torsion wire that suspends the 
>  magnets is also temperature sensitive like the magnets but I don't know 
how 
>  their magnitudes compare or if they add (it would be nice if they canceled 
>  !!). Can somebody figure this one out? A magnet 1/4 inch diameter and 4 
>  inches long is suspended on a 15-inch length of high carbon steel music 
wire 
>  .008 inches in diameter. The torsion wire is twisted so the magnet points 
>  east and west instead of north and south as the magnet would like to do. 
>  Which direction will a temperature change rotate the magnet and how much 
is 
>  due to the magnet and how much to the torsion wire?

    [S/G seismographs and the Lehman ones which have a moving magnet can pick 
up changes in the Earth's magnetic field.] 

    Your magnetometer magnet naturally points N/S magnetic. If your torsion 
suspension rotates it through 90 Deg, all of the couple is due to the 
suspension. The torsional modulus Gm for 1% carbon steel is 8.12 x 10^11 
dynes/cm^2 and the temperature coefficient tc is 2.6 x 10^ -4. Gt = Gm(1 - 
tc(T-15)), where T is the temperature in Deg C.. If the magnet lost field 
(actually magnetic moment) at the same rate as the wire lost torque, the 
rotation would be constant, but even with Alnico 5 at -2.3 x 10^ -4, the 
'fit' is not perfect, so let's consider the problem a bit further.
    The normal N/S horizontal field daily variations may be 40 to 70 nT in a 
field of 0.185 oersted = 18,500 nT. Minor storms may produce 50 to 100 nT, 
major storms 500 + nT. The difference in the magnet strength and torque 
coeffs. is 0.3 x 10^ -4 or 0.55 nT / C Deg. indicated, which really isn't too 
serious. 
    With a Nd-B-Fe magnet at - 0.09% / C Deg., the difference is 6.4 x 10^ 
-4, so the error is 11.8 nT / C Deg. A 4 C Deg daily temperature change will 
give about the same effect as your daily field change. Unless you monitor and 
correct for the temperature changes quite accurately, you won't know the 
difference. (Groan!) The actual angle change that you see is best found by 
measuring how far beyond 180 Degs you have to rotate the top of the wire 
suspension to get a + to - 90 Deg magnet change. To calculate it you would 
need to know the pole strength of your magnet. 
    The damping vane MUST be totally submerged in oil, otherwise you get 
erratic magnet movement due to changes in the surface tension of the oil.
    The CdS photocells are non linear, have quite large differences in 
sensitivity for any two cells and are quite temperature sensitive. Getting a 
well matched pair may be a problem. Check that the cells you plan to use are 
NOT MAGNETIC! Some have magnetic wires / housings. CdS cells usually have a S 
shaped photo gap, so using a knife edge shadow gives a S shaped response, at 
best! You really need to put a diffusing screen between the shutter edge and 
the cell. Large area Si photodiodes used in current mode would be a very much 
better choice and don't cost much more.

    Regards,

    Chris Chapman
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