The balancing scale is one of the oldest weighing instruments known to man. It dates back to the people in Mesopotamia and Egypt during the fifth millennium B.C.E. The earliest forms of balances were made of crude materials, such as straight pieces of wood suspended by a cord passing through the center. Holes were pierced at the ends of the wood and suspended scale pans were attached at both ends. Egyptians, Greeks and Romans later improved the design.
Balancing scales are widely used in industry, from food service and processing to retail, hospitality, automotive, engineering and construction.
The simplest type of balancing scale uses a lever. A uniform bar, called a beam, is suspended at the center and is supported on a knife-edged fulcrum. Two sets of pans with equal weights are suspended from both ends of the beam, with equal distance from the fulcrum. The object to be weighed is placed on one pan, and standard weights are placed on the other until the beam is balanced. This form of measurement relies on the force of gravity on the center of the beam. Some common examples of equal-arm balances include money scales, postal scales, kitchen scales and apothecary scales. Equal-arm balances are not convenient for weighing large objects.
This type of scale is made with one arm of the balance much longer than the other. The shorter arm supports the pan while the longer arm supports the weights that slide to obtain the balance. An object to be weighed is placed on one end of the beam, while a small known weight is moved out along the longer arm until balance is achieved. A common example is a steelyard, used by ancient Romans and Chinese, to weigh large objects.
This type of balance uses the relation between the applied load and the deformation of a spring. A coiled spring is fixed to a support at one end, and a hook is attached to the spring. An object to be weighed is placed on the hook, and the spring stretches in proportion to the weight of the object. The attached pointer and graduated scale gives the weight reading.