The lever is free to rotate on the fulcrum; at its ends are two brass plates suspended from strings. The scale is unbalanced, even without a weight on one of the two plates, because the lever’s fulcrum is not central and thus the distances between it and the points of attachment of the plates are unequal. A scale divided into inches engraved on the arm makes clear the fulcrum’s non-central position.

Equilibrium is thus obtained by adding different weights: the weight placed on the plate on the shorter arm is balanced by a heavier weight placed on the other plate.
Device on display.