Equal-arm Beam Scales
Right hand side, you see a few pictures
of Beam Scales.
Touch a picture, if you want to see some details.
These scales have equal arms with the pan hanging below the beam, on cords, chains or rods. Weighing is achieved by placing known weights in one pan to balance the load in the other pan, and is correct when the beam is horizontal, in almost all examples. The vast majority of these scales are very similar, although they in size from 50mm to over 3m in beam length ( 2" to over 9ft ).
look also to some nice pictures of scales from OTHER collections