A weighing scale is an instrument used for measuring the mass or weight of an object. These devices can be mechanical or electronic. In most cases, scales measure the weight of the object, that is, the force with which the earth is pulling the object towards its centre. Mass, on the other hand, is the amount of matter in the body. However, since the weight and mass of a body are proportional, scales are usually calibrated to give output in mass units.
Center Beam Balance
This is the oldest known weight measuring device and one of the most accurate. This simple device consists of two pans attached to the opposite ends of a lever supported by a fulcrum in the centre. The weight to be measured is placed in one of the pans and known standard weights in the other until a balance is achieved. This balance is indicated by the levelling of both pans with the lever being horizontal. The unknown weight is then calculated by simply summing up the known weights used.
Off-centre Beam Balance
This is a more sophisticated balance involving a lever with a fulcrum quite close to one end rather than in the centre of the beam. A pan is attached to the end close to the fulcrum, and usually a single slide of known weight is on the section of the lever on the other side of the fulcrum. This scale uses the principle of moments to calculate the unknown weight using the known. When in equilibrium, the product of both the weights with their respective perpendicular distances from the fulcrum--called their moments--are the same. This balance thus requires a little technical background on moments.
Hanging Spring Scale
Often seen in produce stores, these scales consist of a spring enclosed in a frame, and a hook attached to one of its ends to suspend the weight to be measured. Whenever a spring is stretched, its extension is proportional to the force applied, which is the unknown weight in this case. The frame is calibrated using known weights to show the weight measurement depending on the stretched length of the spring. Although widely used for measuring mass, these scales measure weight and can be used to calculate differences in earth's gravitational force at different heights above sea level. A limitation of these scales is a confined range of weights measurable by the device.
This device is most commonly used to measure the mass of human beings, often misquoted as their weight. These are nearly identical to the hanging spring balance in the physics principle used, but calculations are based on a spring's compression rather than extension. The spring's compression is proportional to the force compressing it--the weight of the person standing on it. These instruments have limitations similar to those of the hanging spring scale and are similarly affected by the force of gravity, which varies depending on elevation above sea level. The spring can also lose its calibration and become compressed permanently if used by a person weighing more than the maximum range.
Commonly used as shelf-top scales with a digital output, these electronic measuring devices are also based on the spring scale but employ the use of an electrical strain gauge to measure the deflection of the beam which supports the unknown weight. These devices are highly accurate and can be used for measuring small weights, as in bits of jewellery, or large weights, such as heavy trucks, depending on the strain gauge used.