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 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.