Equal arm balance

Using an equal-arm balance (see Fig. 1), the unknown mass of an object can be deterrnined by placing it ia one pan, and adding test weights to the other until the beam balances. The result wHl be the same regardless of location because the object and the test weights ate subject to the same value of. Any scales that measure an unknown mass by comparing it with a known mass (with or without a lever system), wHl be unaffected by variations in g. 

Buoyant Effect of Air. Weighing operations performed m vacuo are not affected by buoyancy forces. An object in air, however, is subject to a buoyancy force that is equal and opposite to the gravitational force on the mass of air the object displaces (10). If the equal arm balance of Figure 1 is in balance with a test weight of mass, in one pan, and material of mass, m, in the other, m = m if they have the same density. If the densities are different, then the buoyancy forces acting on each pan affect the result. Taking moments about the center pivot point gives... 

Balance Design. The principle underlying the design of technical chemical and analytical equal-arm balances is the same. A metal beam (equal-arm lever) is provided with three knife edges—two at its ends and one at its middle (Fig. 22b). 

The lever arm balance compares the mass of an object to the mass of calibrated weights. There are two major types of lever arm balances. The equal arm balance [see Fig. 2.23(a)] works by directly comparing a known weight to an unknown weight, both of which are placed an equal distance from the fulcrum. Once in balance, there is a one-to-one ratio (of the weights) between the unknown and known weights. A well-made balance of this type is capable of high accuracy and preci-... 

The analytical balance has undergone a dramatic evolution over the past several decades. The traditional analytical balance had two pans attached to either end of a lightweight beam that pivoted about a knife edge located in the center of the beam. The object to be weighed was placed on one pan sufficient standard masses were then added to the other pan to restore the beam to its original position. Weighing with such an equal-arm balance was tedious and time consuming. 

Equal-arm balance An analytical balance equipped with a beam that supports two pans equidistant from the ftilcrum—one for the load, the other to accommodate an equal mass of known weights. 

An image commonly used for this competition is the relationship between things on the right and left pans of an equal-arm balance scale (or seesaw) (Fig. 4.1). The direction in which the scale tips depends solely upon the sum of the weights G rai each side of it. Even negative weights are allowed if the objects floating upward (maybe balloons) are attached to the scale. 

The principle of a mechanical balance is based on comparison of mass. The mechanical beam balance may be present in the pharmacy as a back-up. The equal arm balance is three-knife yoke balance (see Fig. 29.3). The object to be weighed is placed on one scale and it is brought into balance with different calibrated weights on the other scale. 

It is possible to make a measurement by having the information reside entirely in nonelectrical domains. For instance, the determination of the mass of an object using a mechanical equal-arm balance involves a comparison of (he mass of the object, which is placed on one balance pan. with standard masses placed on a second pan. The information representing (he mass of the object in standard units is encoded direcih by (he experimenter, who provides information processing by... 

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