How Are Scales & Balances Constructed?

How Does a Balance Scale Work?

The balance scale was the first instrument ever invented for measuring weight and mass. It consists of a beam and two scales, or flat pans, suspended from either end. The beam is set on a point so that both arms are of equal length. An object of unknown weight is set in one scale, and then weights of known size are placed in the other until a balance is achieved.

Equilibrium

Since the weight in a balance scale is determined by achieving equilibrium between the two scales, the point at which the beam is balanced is of critical importance. Friction, for example, could throw off the measurement by a considerable degree. For this reason, the fulcrum of the beam is placed on a knife-edge to reduce the force of friction to a necessary minimum.

The Slider

The balance scale works by placing pre-measured weights into the opposing scale to determine the weight of a given object. However, few weights ever fall neatly into the gradations of a set of scale weights. For this reason, balance scales come equipped with a slider weight, which can be adjusted to make the last bit of counter-balance weight to reach a precise determination.