Right hand side, you see a few pictures of Steelyards.
Touch a picture, if you want to see some details.

A steelyard has a beam with unequal arms. The long arm has a weight suspended from it, which can be moved along the arm until equilibrium is obtained, the weight being indicated by graduations on the beam. The load is suspended from the short arm of the beam.
Essential features of a steelyard are that the position of the weight along the beam varies according to the load, and the graduations are equally spaced. A major advantage of the steelyard is that only one weight is required to balance a whole range of loads.
There are many variations on this principle, including ones with weights that slide on the beam, weights that swing over into two positions, and beams which slide and act as weights. They were used for shop scales, person scales, large warehouse scales and weighbridges.

More Steelyards under Letter Scales / Steelyards