Testing the position of the field point relative to the mass center of the prism (resulting from the computation of sm) can be avoided in Plouff's equation (3) by assuming
g=γρ∑L=1n{SpA[|z2|−|z1|+z2. . .]},
where z1 and z2 are the vertical coordinates (not their absolute values) of the top and bottom, respectively, of the prism. An absolute value is necessary for the computation of the angle; i.e.,
spA=sgn (x1y2−x2y1)cos−1[|x1x2+y1y2|/(r1r2)]         =π/2 sgn (x1y2−x2y1)−tan−1[(x1x2−y1y2)/(x1y2−x2y1)].
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