The everyday, household spring balance looks like a cylinder with a hook at one end and a ring at the other. There are five parts to the scale. The ring at the top is to hold or hang the scale. The spring deflects according to the weight of the object. This does the actual job of weighing. Attached to this spring is the pointer that moves along the graduated measurement on the face of the scale. The graduated markings on the face indicates the weight. At the bottom is the hook upon which the object to be weighed is attached.
In a simple spring scale, the spring extends downwards by the pull of the object on the hook. Measurement units on the face show the weight of the object. The scale in a grocery store exemplifies a spring scale. The attached pan makes for ease of use. The customer places the produce in the pan and reads the weight.
Spring scales also work on the principle of compression. A bathroom weighing scale is a good example. The weight of the person or the object compresses the spring.
How do the markings on the scale relate to the weight? In other words, how does the scale know how heavy the object is? Every spring has a spring constant, or a proportionality constant that relates the pull on the spring to the distance it extends. Hooke's Law governs the distance the spring will extend. It states: The power (sic.) of any springy body is in the same proportion with the extension. Simply put, there is an observable relationship between the force applied to a spring and the length it extends.
There are two types of measuring scales: the spring scale measures the gravitational force that acts on an object, and the balance scale that measures the mass of an object. We use both scales to measure the weight of objects, but there are scientific differences. A spring balance is dependent on gravity, which is not constant throughout our planet. It can vary locally as much as 0.5%. The beam balance is gravity independent. It will give the same reading, no matter where in the universe you place it.
Calibrate a spring scale to its specific location. Recalibrate it if moved. Temperature differences affect the elasticity of the spring.