A balance is an instrument employed for determining the quantity of any substance equal to a given weight. Balances are of various forms; in that most commonly used a horizontal beam rests so as to turn easily upon a certain point known as the centre of motion. From the extremities of the beam, called the centres of suspension, hang the scales; and a slender metal tongue midway between them, and directly over the centre of motion, indicates when the beam is level. The characteristics of a good balance are: 1st, that the beam should rest in a horizontal position when the scales are either empty or loaded with equal weights; 2nd, that a very small addition of weight put into either scale should cause the beam to deviate from the level, which property is denominated the sensibility of the balance; 3rd, that when the beam is deflected from the horizontal position by inequality of the weights in the scales, it should have a tendency speedily to restore itself and come to rest in the level, which property is called the stability of the balance.
To secure these qualities the arms of the beam should be exactly similar, equal in weight and length, and as long as possible; the centres of gravity and suspension should be in one straight line, and the centre of motion immediately above the centre of gravity; and the centre of motion and the centres of suspension should cause as little friction as possible. The centre of motion ought to be a knife-edge; and if the balance requires to be very delicate, the centres of suspension ought to be knife-edges also. If the balance have no tendency to one position more than another, when the scales are either loaded, empty, or off altogether, it is a proof that the centres of gravity and motion coincide, and the remedy is to lower the centre of gravity. If the beam is disturbed by a small addition of weight to either scale, and exhibits no tendency to resume the horizontal position, we may infer that the centre of gravity is above the centre of motion.
If it require a considerable excess of weight to deflect the beam from the level, we may infer either that there is too much friction at the centre of motion, or that the centre of gravity is too low. If two weights are found to be in equipoise, one being in each scale, and if, when that which is in the one scale is put into the other, there is no longer equilibrium, then we may infer that the arms of the beam are of unequal lengths. Eor purposes of accuracy, balances have occasionally means of raising or depressing the centre of gravity, of regulating the length of the arms, etc, and the whole apparatus is not unfrequently enclosed in a glass case, to prevent the heat from expanding the arms unequally, or currents of air from disturbing the equilibrium.
Of the other forms of balance, the Roman balance, or steelyard, consists of a lever moving freely upon a suspended fulcrum, the shorter arm of the lever having a scale or pan attached to it, and the longer arm, along which slides a weight, being graduated to indicate quantities. It was commonly used for weighing loaded carts, for luggage at railway-stations, etc. A variety of this, the Danish balance, has the weight fixed at the end of the lever, the fulcrum being movable along the graduated index. The spring-balance shows the weight of articles by the extent to which they draw out or compress a spiral spring. It is of service where a high degree of exactness is not required, and finds application in the dynamometer for measuring the force of machinery. Research Balance
Abbreviations
SciTech
Research Balance