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I will have to measure very small EM forces (on the order of 10^-6...10^-5 Newtons) acting on a small copper sphere or plate (of about 0.05 kg).

I could hang the copper object with a long thread and use a micro-Newton sensor but I can't find any vendor that gives prices without contact so i guess these are very expensive equipment for me.

Then I thought maybe I could tie a lightweight rod in the center to the thread with 2 identical copper objects on the ends with balance.

It would be very similar to the Coulomb's Torsion Balance set up.

But I'm confused about what to do with this setup.

For example, there could be 10^-6 Newtons from 0.5 meters distance (half the rods length) to center. Would such a small moment produce any measurable angle of twist in a common silk thread? Even if it did, I wouldn't be able to calibrate the scale and know how much force there is. enter image description here

Or should I try to add a second but much shorter rod and try to measure the "amplified" force with less sensitive force sensor?

enter image description here

Or is there a more common method I'm not aware of?

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    $\begingroup$ This recommends nylon monofilament . Says natural fibres twist with humidity cahnge. $\endgroup$ – Keith McClary Nov 13 '16 at 22:47
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Rewritten in response to comments.

The torsion balance is often used to measure gravitational attraction of small objects.

You could make a torsion balance with your copper sphere. Add a metal sphere B as shown in the diagram. Calculate the gravitational attraction. Measure the deflection. Use this to calibration your balance.

If gravity is weaker than the forces you are interested in, charge the spheres and calculate the electrostatic attraction/repulsion.

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