How Does a Balance Scale Work?

Why a Balance Scale?

A balance scale is used to perform precision measuring and weighing. Other types of scales are affected by gravity and, depending upon geographic location, can vary in their results. Balance scales do not have this issue because they are balanced, which implies both sides of the scale are affected exactly the same. Balance scales are also referred to as beam balances or laboratory balances. The scales that the statue of Justice holds are traditional balance scales consisting of a horizontal beam (lever) with a scalepan, or scale of equal size and weight, hanging from each end of the beam.

Parts of a Balance Scale

To ensure precise measurements on a balance scale, the lever or horizontal beam has at its center a fulcrum or pivot point. The fulcrum was created in the form of a V-shape that keeps the beam in a precise place. A mass of known weight, usually in metal bars, is placed on one scale and the object to be weighed is placed on the other. Bars of known weight are added or removed until the two sides balance. On some scales the object being weighed is not placed in a scalepan but is hung on either side of the scale from the beam.

Types of Balance Scales

The scale upon which a person is weighed in many doctor's offices is also a balance scale. This is not a traditional balance scale in that there are no equal scalepans to place a patient on. Instead this balance scale is an off-center beam. This beam uses a sliding style weight, called a poise, that is positioned across a calibrated arm or scale. Some balance scales have the horizontal beam at the bottom of the scale with the pans sitting on top. Larger loads can be measured on a platform that floats on a cantilevered type beam. Bathroom scales are a smaller version of this type of scale. Accuracy is always off slightly in these scales because additional pivots and mechanics complicate the calibration.

Balance Scale Errors

Air can be the biggest culprit of error in balance scales, and this is why high-end balance scales that require precision and complete accuracy are used inside a vacuum. Other errors can result from items that move. When placed for weighing, these moving items don't allow for the scale to completely balance. Dust that can settle onto the object being weighed, uncalibrated scales, evaporation, condensation and even static electricity can all cause errors in a balance scale.