Dear Sean Morrisey,
    
    Radio Shack no longer list Q>> a 10-inch dual coil "sub-woofer" # 40-1349 
with a 20 ounce magnet. The strength of this large ceramic magnet is listed 
at 7500 gauss in the annular gap of the coil. 
    I can only find figures for the total flux BL, not the field strength. 
What is the coil dia., BL value and power rating of your speaker please? I 
doubt if I would get much cooperation if I went into a shop which sells 
speakers and asked if I could check if passing one amp through a speaker 
would lift 100 gm.

    OK, so atmospheric pressure changes are a problem. Why not extend the 
beam on the other side of the hinge and fit a light rigid balancing capacity? 
Something like an Al beer can.  The standard cans are 350 ml, but you can 
also get 150 ml cans - I calculated the volume of just the lead on it's own 
is about 50 ml. You drill a couple of 1/8" holes in the top of the can, 
remove the beer, coke, whatever, hiccup, wash out and dry the can and then 
epoxy a couple of bits of another can over the holes. This should enable you 
to get compensation to ~ 1%. Wouldn't that be good enough?

    Is a mechanical weight adjustment really necessary? The maximum offset 
current through the magnet coil is less than 75 micro amps at balance. 
Couldn't you just provide a FET current source to give just enough torque to 
re-balance the beam again? The voltage offset on the coil would be milli 
volts at most, constant and of such a high impedance that the other sources 
would not 'see' it. You don't seem to use the 0 to 2 V range from your power 
supply lines, so a 1.2 v max. current source would fit in nicely. OK, the 
magnet field changes slightly with temperature, but the error% would only 
show on the offset bias and should be quite small. Another alternative would 
be to have a sealed shaft driving a cord around pulleys, fitted with a plate 
with a fairly wide slot to pull a weight along the beam / leave it free when 
correctly positioned.

    You mentioned that many manufacturers use capacitative transducers. What 
do the capacity elements look like, please? Common ones are two flat plates 
with a small variable separation between them - three plates, two fixed and 
the centre one moving so that the separations alter - two pairs of plates 
with a common third plate sliding between them (differential capacitor). How 
do they keep the accurate alignment necessary for a capacitative transducer? 
The main problem, as I see it, would be getting the very high sensitivity and 
stability required. From experience, getting a volt per thou isn't a problem, 
but getting another factor of 25 in both sensitivity and in stability sounds 
rather more challenging. 

    Regards Chris Chapman.

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