The 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

Hooke's law

In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which 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.