A spring scale is a scale which measures weight by determining the deflection of a spring. As long as the spring in good condition, the spring scale will be reasonably accurate. The big advantage to this type of scale is that it is cheap and very easy to use. The primary disadvantage is that spring scales are not always very accurate, and they are subject to wearing down over time, which causes the scale to register weights incorrectly.
The design of a spring scale takes advantage of the fact that if one knows the distance a spring is extended from a resting state, it is possible to determine the amount of force needed to extend the spring that far. As long as the metal in the spring is not stretched beyond its elasticity, it should give a reasonably accurate reading, which can be registered by the scale in a variety of ways, depending on the design.
Spring scales must be carefully calibrated in order to work properly. The properties of the individual spring in the scale must be known by the manufacturer, and the scale needs to be routinely checked to confirm that it is accurate. If the spring is overloaded and worn out, it will skew the scale results, creating an inaccurate reading. The spring can also break, causing the scale to fail altogether.
A classic example of a spring scale can be found in the produce sections of many supermarkets. With this type of scale, people put objects into a tray held below the spring scale, and the weight of the objects causes the spring to expand. The scale's dial reflects the amount of deflection experienced by the spring, indicating how much force was exerted on the spring to cause it to move. Spring scales are used in many settings where people need to quickly weigh things, and pinpoint accuracy is not necessarily needed.
Several companies sell portable spring scales with hooks which can be tremendously useful. To use such a scale, the scale is hung from a fixed object, and the object being weighed is attached to the hook. This lightweight scale design is useful for people doing field work, as it allows them to weigh objects in the field without needing to carry around a large scale. These small spring scales are also sometimes used in the classroom to educate children about weights and measures, and to introduce students to the basic properties of springs.