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The spring balance (also known as spring scale) apparatus is simply a spring fixed at one end with a hook to attach an object at the other. It works by Hooke's Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position. Therefore the scale markings on the spring scale are equally spaced.
Spring scales can be calibrated for the accurate measurement of mass in the location in which they are used, but many spring scales are marked right on their face "Not Legal for Trade" or words of similar import, due to the approximate nature of the theory used to mark the scale. Also, the spring in the scale can permanently stretch with repeated use.
If two spring scales are hung one below the other in series, each of the scales will read the weight of the body hung on the lower scale. The scale on top would read slightly heavier due to being stretched by the weight of the lower scale.
Spring scales come in different sizes. Generally, small scales that measure newtons will have a less firm spring (one with a smaller spring constant) than larger ones that measure tens, hundreds or thousands of newtons.
Spring scales can be used in physics and education as basic accelerometers, but its main uses are industrial, especially related to weighing heavy loads such as trucks, storage silos, and material carried on a conveyor belt. Spring scales are used when the accuracy afforded by other types of scales can be sacrificed for simplicity, cheapness, and robustness. A spring balance measures the weight of an object by opposing the force of gravity with the force of an extended spring.
The first spring balance in Britain was made around 1770 by Richard Salter of Bilston, near West Bromwich. He and his nephews John & George founded the firm of George Salter & Co., still notable makers of scales and balances, who in 1838 patented the spring balance. They also applied the same spring balance principle to steam locomotive safety valves, replacing the earlier deadweight valves.