Equal-arm balance

An equal-arm balance is a weighing instrument with a pan at each end of a beam and a pivot between them. An object in one pan is compared with weights in the other. When the system balances, the masses can be compared.

The term equal-arm identifies the construction. Not every older scale with a pan uses this arrangement. A steelyard or bismer has unequal arms, while a spring scale uses a different measuring principle.

Balances vary in size and construction. Science Museum Group documents an equal-arm balance with components including brass, steel, iron and mahogany. The Metropolitan Museum of Art holds another set with pans, weights and a box, catalogued as British and probably dating from the eighteenth century. Material or an accompanying box alone does not establish an object's date or maker.

For assessment, record the beam, suspension, both pans, weights and any box separately while preserving their association. Photograph markings and note missing parts. An old balance moving or reaching equilibrium does not demonstrate its measurement accuracy. Record condition before cleaning or repair.

Browse OBA's older practical objects. If you wish to sell a complete or incomplete set, send photographs and measurements through our buying enquiries page.