In the construction of the B., it is a matter of importance to have the sensibility inde pendent of the amount of weight in the scales, so that, when heavily loaded, a small weight will produce the same inclination as when not loaded at all. This condition is implemented, as we have alreadyshown, when the three knife-edges are kept in the same straight line. If the line joining the two terminal knife-edges lie below the point of sus pension, then the center of gravity of the equal weights corresponding with the middle of that line, will, upon the turning of the beam, be forced from below that point, and will accordingly have it tendency to resume its former position. The equal weights thus counteract to some extent the effect of the additional weight, in causing the beam to incline, and their influence in this way will be all the greater as fluty themselves increase. When n B. is too heavily loaded for its strength, the three knife-edges, although previously in a line, do not retain that position, for the arms of the beam yielding to the pressure, cause the terminal knife-edges to sink below the one in the middle, and the knife-edges themselves losing their shape under the pressure, the sensibility is considerably diminished.
When it B. is very the beatn keeps oscillating for a considerable time from one side to the other of the position in which it finally settles. Although such an instrument may be useful for physical and chemiCal experiments, it is not serviceable for the pur poses of ordinary life, where minute quantities of the substance to be weighed are of little vnlue, and where time, and consequently rapidity of indication, are matters of importance. The sensibility of a B. must, therefore, be adjusted to the purpose for which it is designed; sensible balances being employed for weighing finer, and less sensible, or stable balances, for weighing coarser materials. The stability, or the teudency of the beam to come quickly to rest, depends on requirements nearly the opposite of those which conduce to sensibility. In the construction treated of above, the stability increases with the moment of the weight of the beam acting at G round 0, so that it thus increases with the weight of the beam, and the distance of the center of gravity from the point of suspension. The stability is also increased, as already shown, by having the line joining the scale knife-edges below the point of support.
'I here is another form of delicate balance employed in physical and chemical researches. The beam is constricted so as to combine lightness with strength, and rests by a fine knife-edge on an agate plane. It is surmounted by it weight moving on a screw, so that the sensibility may be increased or diminished, according as the weight is raised or depressed. In order that the knife-edge may not become blunted by constant contact with the supporting plane, a cross-bar, with two projecting pins, is made to lift the beam from the plane, and sustain its weight when the balance is not in play. The beam is divided bylines marked upon it into 10 equal parts, and a small weight made of fine wire bent into the form of a fork, called a rider, is made to slide along to any of the divisions. If the rider be. for instance, of of a grain, and if, after the weight of a body is very nearly ascertained, it brings the beam, when placed at the first division next the center, exactly to its horizontal position, an additional weight of 4 of a grain will be indicated. The use of inconveniently small weights is, by this arrangement, to a large extent obviated. As the beam takes some time before it comes to rest, it would be tedious to wait in each case till it did so, and for this reason a long pointed index is fixed to the beam below the point of suspension. the lower extremity of which moves backward and forward ou a graduated ivory scale, so that when the index moves to equal distances on either side of the zero point, we are quite certain, without waiting till it finally settles, that the beam will be horizontal. The same is seen in ordinary balances, only the tongue or index is
above the beam; and according to its deviation on each side of the fork or cheeks by which the whole is suspended, is the future position of the beam ascertained. The finer balances are never loaded to more than a pound in each scale, and when so charged, will deflect with of a grain of additional weight in one of the scales, or will turn, as it is technically called, with of the load. The finest balances turn with of the load, and some have been constructed which turn with much less. Even with the best achievethents of mechanical skill, no B. can be made whose arms are absolutely equal; and to remedy this defect, the method of double-weighing is resorted to, when the utmost accuracy is demanded. This consists in placing the body to be weighed into one scale, and sand, or the like, into the other, until exact equilibrium is obtained, then removing the body, and putting weights or another body in its place which exactly counterbalance the sand. Both being thus weighed in precisely similar circumstances, must weigh precisely the same.
The Roman B., or steelyard (Ger. schnellwage), is more portable than the ordinary balance. It consists of a lever AB, moving round a knife-edge or point at C. The body to be weighed, IV, is put into the scale which hangs from A; and movable weight, P, is made to slide along the longer arm, until the lever AB remains horizontal. The weight of Wis then read off from the division at which P rests. If the lever lie horizon tal when unloaded, then equal weights at equal distances from C will balance each other, so that when IV is balanced by Pat a distance front C equal to AC, the two are of equal weight; but if equilibrium take place when P, say, is ten times as far from C as A is, then IV will be ten times the weight of P; and the same holds for any intermediate point at which P may stand. W weighing as many times Pas P's arm is a multiple of W's arm. To weigh a body of 10 lbs. by the ordinary B., a counterweight of 10 lbs. is necessary, snaking a total load of 20 lbs.; bat in the case just supposed, 1 lb. balances 10, making a total load of only 11 lbs. The steelyard has, therefore, this advantage over the common B., that the load on the fulcrum, and consequently the friction, is less. On the other hand, however, there is this disadvantage, that the arms of the steelyard bend unequally under the strain of great weights, which in a B. with equal arms cannot, to the same extent, take place. As the steelyard is ordinarily made, the longer arm preponderates when the lever is unloaded, so that thegraduation of the longer arm begins at a point between A and C, and not at C. The Danish B. differs from the ordinary steelyard in having the weight fixed to the extremity of the lever, and the fulcrum movable.
The bent lever B. (Fr. peson, Ger. Zeigere'age), shown in fig.
2, is a lever of unequal arms, A, C, B, moving round the pivot C, having a scale, Q, attached to the shorter arm, AC, and a fixed weight, W, to the longer arm C13. The longer arm ends in a pointer moving in front of a fixed graduated arc. When a body is put into the scale, the pointer rises from the bottom or zero point of the arc, and rests opposite the mark correspond ing to the weight of it. The higher the weight W rises, the longer becomes its effective arm, and the greater must be the, weight it balances. The arc is generally graduated experimen tally,the geometrical graduation being somewhat complicated.
For other weighing apparatus, see SPICING- BALANCE:.