Balance Beam Scale
The balance beam scale (also known as a beam or laboratory balance) was the first mass measuring instrument invented. The balance beam scale in its most basic form consists of a pivoted horizontal lever with arms of equal length – the beam – and a weighing pan suspended from each arm (hence the plural name "scales" for a weighing instrument). To measure an unknown mass’s weight, the unkown mass is placed in one pan, and standard masses of with known weights are added to the other pan, until the beam is as close to equilibrium as possible.
Balance beam scales are used for the precision measurement of mass as their accuracy is not affected by variations in the local gravitational field. (On Earth, for example, these can amount to ±0.5% between locations.) A change in the strength of the gravitational field caused by moving the balance will not change the measured mass, because the moments of force on either side of the beam are affected equally. A balance will render an accurate measurement of mass at any location experiencing a constant gravity or acceleration.
Very precise measurements are achieved by ensuring that the balance's fulcrum is essentially friction-free (a knife edge is the traditional solution), by attaching a pointer to the beam which amplifies any deviation from a balance position; and finally by using the lever principle, which allows fractional masses to be applied by movement of a small mass along the measuring arm of the beam, as described above. For greatest accuracy, there needs to be an allowance for the buoyancy in air, whose effect depends on the densities of the masses involved.
The original form of a balance consisted of a beam with a fulcrum at its center. For highest accuracy, the fulcrum would consist of a sharp V-shaped pivot seated in a shallower V-shaped bearing. To determine the mass of the object, a combination of reference masses was hung on one end of the beam while the object of unknown mass was hung on the other end (see balance and steelyard balance). For high precision work, the center beam balance is still one of the most accurate technologies available, and is commonly used for calibrating test weights.