BALANCE (Latin, bis, °twice,* and low, a °dish," or °pan"), an instrument for deter mining the mass of a body by comparison with a senes of other bodies (called °weights)) whose masses are lcnown. The term is often applied, though somewhat incorrectly, to the familiar instruments in which the weight of a body is determined by observing the extension that it can produce when acting upon a spring whose extensibility has been previously deter mined by direct experiments with known weights. The °spring balance)) is useful in the ordinary affairs of life, where high precision is not essential; but it is seldom employed in accurate scientific work, since it is liable to errors that cannot be eliininated or allowed for —errors that are small enough to be neglected in conunercial transactions, but quite intolerable in refined laboratory work.
The °lever balance" consists essentially of a lever (q.v.) having arms of known lengths. The mass to be determined is suspended at the extremity of one of the arms, and the laiown masses (or weights) are suspended from the extremity of the other one, their number and size being varied until, after repeated trials, a perfect equilibrium, or °balance," is attained. If the two arms of the lever are equal, the mass of the body under examination is then equal to the sum of the masses of the weights that are balanced against it In many cases (for example, in the familiar °platform scales") the arms of the lever are intentionally made very unequal, the object to be weighed being suspended from the short arm of the lever, while the weights are suspended from the long arm. To determine the mass of the object it is then necessary to multiply the sum of the masses of the weights by the ratio of the long arm to the short one; but in practical work this calculation does not need to be performed, because the instrument is graduated by the maker so that all necessary allowance for the difference in the arms has been made, and the readings give the corrected mass directly. In many cases the balances (or °scales))) used in conunerce are constructed so that equilibrium is attained by varying the length of the lever arm rather than by varying the load at the extremity of that arm; but the fundamental principles involved are the same in all cases; and are set forth in detail in the article LEVER (q.v.).
In the °precision balance" of the chemist and physicist, the lever (called the ((beanill consists of a light but strong and rigid frame work, usually made of brass or bronze, and having a shape somewhat like that shown in Fig. 1. It is supported by means of a wedge shaped piece of steel, technically known as a °knife-edge," which is hardened and ground to a sharp and accurately straight edge, and which rests, when the balance is in use, upon a flat slab of agate, or other hard, smooth substance, in such a manner as to leave the beam free to tip one way or the other, with practically no frictional resistance. (The agate slab is sug
gested bv the dotted contour, k, in the figure; the pillar that supports k being omitted for the sake of clearness). 1Cnife-edges similar to the central one, but with their edges directed up ward instead of downward, are provided at the respective ends of the beam (as shown at A and B) for the support of the pans (only one of which is shown) in which the masses to be compared are placed. The three knife-edges, A, B and C, must be made with great care, and must be set in position so that they shall be accurately parallel to one another. They must, moreover, have their edges all in the same plane, so that a straight line joining any two points in the edges of A and B will like wise pass through the edge of C. The two arms of the beam should also be precisely equal, so that C is exactly half way between A and B. P is a pointer whose free end travels over a graduated scale, so as to indicate the extent of the oscillations of the beam as it swings to and fro on the central knife-edge C. When the beam is horizontal, its centre of gravity (G in Fig. 2) should lie in the same vertical line, ab, with the central knife-edge. Whether this condition is fulfilled or not is easily shown by removing the scalepans and allowing the be_am to come to rest. It can only be in equilibrium when its centre of gravity is directly below the knife-edge C; so that if it comes to rest in a horizontal position it is evident that the condition specified above is sensibly realized. If, on the other hand, the beam, when freed from the pans, comes to rest with its right-hand end lower than the left hand one, it is evident that the centre of gravity of the beam is too far to the right, as is indi cated by the point g. The better makes of balance are provided with an adjusttnent to correct an error of this sort. This adjustment may ta1ce the form of a fine screw-thread carry ing a nut, as suggested at E. If the nut be caused to approach B, the centre of gravity of the beam (considering the nut as a part of the beam) will thereby be shifted toward the left, and after a number of trials the point g may be made to coincide with G, so that the beam, when free from the pans, comes to rest in a perfectly horizontal position. If it does not re main horizontal when the pans are suspended in their proper places, then it follows that one of the pans is heavier than the other; this defect is easily remedied by the use of a light counter poise in connection with the lighter pan, or by removing a small portion of the material of the heavier one.