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BTW, Stong's Amateur Scientist (Sci. Am. Sept. '63)
gives instructions for a Mitchell torsion balance; it's large -- suspension 55" (#36 music wire**) arm, 41" weights (masses) 46 oz. juice can lead filled, and 100 gm. Both cylindrical -- like the recent rather more accurate one using W. Spheres are best, but diff. to accurately machine, no?
** I remember reading another one using 1/4" mag. tape.
bc notes included an electrostatic shield -- much larger constant!