Traditional Balance
A beam balance uses a horizontal arm known as a "beam" to compare two weights. This is in contrast to a scale that directly measures a weight. The earliest form of balance consisted of a single beam that with a pivot in the middle to provide two arms of equal length. Each arm has a weighing pan suspended from the end such that the pans have equal weight. An object with an unknown mass is placed in one of the weighing pans and known weights are placed into the other weighing pan until the beam is as evenly balanced as possible.
Precision Balance
A balance can achieve greater accuracy by placing a known mass on the beam. This type of balance doesn't have a pivot and instead allows the known mass to slide along the beam to locate the equilibrium point. This design provides a continual correction to the known weight rather than making incremental corrections, as is the case with the traditional balance.
Additional Improvements
The accuracy of a beam balance can be further improved by making refinements in its basic design. The friction of the fulcrum may be minimized by using a knife edge, which is essential for balance beams that make extremely precise measurements. The beam may also have a pointer attached to its free end to indicate the equilibrium point more clearly. Precision beam balances may also need to account for differences in the buoyancy of air.
Advantages
A beam balance is generally more accurate than a spring scale because the measurements of a beam balance aren't affected by changes in gravity. This is because the weight on each side of the balance is affected equally by gravity. This advantage over a scale is significant because the gravitational field on the surface of the Earth varies according to the specific location. Precision measurements are typically made with some type of beam balance.
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