A spring balance uses the extension of a spring to measure a force. Spring scales, such as are often found in supermarkets, are a common type of spring balance. The spring balance relies on Hooke's Law, which states that, for any given increase in force, an elastic object will deform proportionately.
A spring being stretched is acted upon by a force. Hooke's law states that, if the force is doubled, the amount of stretch in the spring doubles as well, at least in relatively low levels of force. Spring balances are either read directly, with gradations marked on a transparent container, or via a mechanism which turns a rotary indicator. Spring balances typically use coil springs, but any elastic object can serve in a spring balance.
Spring balances are only valid up to a certain level of force. Beyond this level, the force exceeds the elastic resistance of the spring and the object deforms more than the force would otherwise indicate. To avoid this, and to take into account variations in individual springs, a spring balance must be carefully calibrated. This is done using objects of known weight, or that otherwise exert known forces, and then adjusting the reading mechanism so the measurement is accurate.