A balance beam scale is a type of scale that uses a system of sliding weights that move on graduated beams. A lever system connects the beam assembly to the pad where the object to be weighed is placed. The beam assembly has a pointer at one end that when properly balanced is centered on a fixed point or bracket. By gradually adjusting the sliding weights on the beams, the pointer can be centered and the weight of the object can be determined by reading the markings on the beams.
The digital age has made available very precise digital scales, which have become more and more affordable in recent years. Balance beam scales are still common, however, as they can also be very precise; some models will measure masses at a level of precision of two or three decimal places. They are purely mechanical and do not need a power source.
The basic design of the balance beam scale goes back centuries. These types of scales are derived from simple balance scales, the earliest known examples of which date back to around 2000 B.C. These scales consist of a single beam supported in its exact center with a pan suspended from each end. The item to be weighed is placed on one pan and objects with a known weight are added to the other pan until the scales balance.
Many doctors offices use balance beam scales. This type of balance beam scale is designed to weigh a person, who stands on the pan. Sliding weights are adjusted on the beams until the pointer is steady in the center of the bracket and the subject's mass is read on the beams. The configuration is slightly different than table-top models, but the general design and operation of both types is the same. A table-top balance beam scale is a common instrument in classrooms, laboratories, assay offices, jewelry shops, and pharmacies.
Balance beam scales measure the mass of an object, rather than its weight. The difference is negligible on the Earth's surface and the two terms are usually used interchangeably without confusion. If one were to weigh someone on the moon, however, their weight would be approximately one sixth of their weight on Earth. A balance beam scale would give the same result both places.