The balance (also balance scale, beam balance and laboratory balance) was the first mass measuring instrument invented. In its traditional form, it consists of a pivoted horizontal lever with arms of equal length – thebeam – and a weighing pan suspended from each arm (hence the plural name "scales" for a weighing instrument). The unknown mass is placed in one pan and standard masses of known weight are added to the other pan until the beam is as close to equilibrium as possible. In precision balances, a more accurate determination of the mass is given by the position of a sliding mass moved along a graduated scale. Technically, a balance compares weight rather than mass, but, in a given gravitational field (such as Earth's gravity), the weight of an object is proportional to its mass, so the standard "weights" used with balances are usually labeled in units of mass (g, kg, etc.).
Laboratory balances are defined by high levels of accuracy and precision in analytical testing and quantitative analysis. Used in a variety of applications, they are divided into five subcategories according to their readability:
- Precision balances (>= 0.001 g)
- Analytical balances (0.0001 g)
- Semi-micro balances (0.00001 g)
- Micro balances (0.000001 g)
- Ultra-micro balances (0.0000001 g)