Weighing scales (usually just “scales” in UK and Australian English, “weighing machine” in South Asian English or “scale” in US English) is a measuring instrument for determining the weight or mass of an object. Weighing scales are used in many industrial and commercial applications, and products from feathers to loaded tractor-trailers are sold by weight. Specialized medical scales including infant medical scales, and bathroom scales are used to measure the body weight of human beings. The name scales derives from the pair of scales or dishes in which objects to be weighed and the weights / masses against which to weigh them are placed. The Oxford English Dictionary defines scales as “Apparatus for weighing. The pan, or each of the pans, of a balance.” Spring balances or spring scales measure force or weight by balancing the force due to gravity against the force on a spring, whereas a balance or pair of scales using a balance beam compares masses by balancing the force of gravity (weight) due to the mass of an object against the force due to gravity (weight) of a known mass. Either type of balance or scales can be calibrated to read in units of force (weight) such as Newtons, or in units of mass such as kilograms, but the balance or pair of scales using a traditional balance beam to compare masses will read correctly for mass even if moved to a place with a different (non-zero) gravitational field strength (but would then not read correctly if calibrated in units of force), while the spring balance would read correctly in force in a different gravitational field strength (but would not read correctly if calibrated in units of mass).
