A spring scale, also known as a spring balance or newton meter, is a type of mechanical force gauge or weighing scale that measures weight or force when an object is hung from its hook. It consists of a spring fixed at one end with a hook to attach an object at the other. The main uses of spring balances are to weigh heavy loads such as trucks, storage silos, and material carried on a conveyor belt. They are also common in science education as basic accelerators. Spring scales can be used to measure weight in a wide range, depending on the stiffness of the particular spring. Small scales with sensitive springs can be used to measure differences of up to a gram or less, while larger ones can measure tens, hundreds, or even thousands of newtons. Spring balances can be calibrated for the accurate measurement of mass in the location in which they are used, but many spring balances 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.