Braby's Balance, or Weighing Apparatus, unites the properties of the bent lever balance, and the steelyard. It has been termed a domestic balance by the inventor, from its being p( culiarly adapted for family purposes, such as ing meat, bread, butter, &c. In the figure A B C is a frame of cast-iron, which has the greater part of its weight towards A, in consequence of its greater thickness at that part. F is a fixed fulcrum, and E II a movable sus pender, which has a scale and hook at its lower extremity. E K G are three distinct points to which the suspender E H may be applied, and to which belong, respectively, the tbree graduated scales of weights f C, c d, a b. When the suspender is applied at G, the apparatus is in equilibrio with the edge A B hori zontal, and the suspender cuts the zero of the scale a b. If a weight be now placed in the scale, the whole apparatus turns about F, and the point on the aide B C descends till the equilibrium is again established. The weight placed in the scale may now be read off from the point where the suspender cuts the scale a b, which registers to ounces, and is adapted to bodies whose weight does not exceed two pounds. If the weight of the body exceeds two pounds, but is under eleven, the suspender is placed at K; and when the upper edge of the balance is horizontal, the weight, or number 2, is found a little to the right of the index of the suspender; if, now, weights exceeding two pounds be placed in the scale, the whole again turns about F, and the weight of the body is shown on the graduated arc c d, which extends to eleven pounds, and registers to every two ounces. If the weight of the body exceeds eleven pounds, the sus pender is hung on at E, and the weights are ascertained in the same manner on the scale/ C to thirty pounds; the subdivisions on this scale are quarters of pounds. The same principle might be extended to weights greater than the above. To prevent mistake, the three points of support, G, K, E, are numbered 1, 2, 3; and the corresponding arcs are respectively numbered in the same manner. When the hook is used instead of the scale, the latter is turned upwards, there being a joint at H for that purpose.
Balance of Torsion. If a piece of very fine wire, silk, or spun glass, ex tended by a weight, be suspended to any fixed point, and then twisted, it will, when released, begin to untwist itself, and, by its momentum acquired in the act of untwisting, will twist in the opposite direction. It will afterwards return, and thus, by a series of oscillations, continually diminishing in amplitude, it will at length come to a state of rest in its original position. Now, if a needle, or an index similar to the hand of a watch, be attached to the lower extremity of the suspended wire, and a circle, having its circumference graduated into degrees, or other equal divisions beneath it, it will form the balance of torsion. To measure small forces, as those of electricity, magnetism, &c., with this balance, they are made to act on one extremity of the index ; and when the force is in equilibrio with the tendency of the wire to untwist, the angle which the index makes with its quiescent position, which is called the angle of torsion, is the measure of the force employed. In the annexed
cut, letabcg represent the graduated circle, and a g the index suspended by the silver wire at the point e.
Suppose any force act upon a so as to move it to b, then will the arcs a b represent the angle of torsion. Sup pose another force to act at the same distance, and move the ball to c, then will the latter force be mea sured by the arc a c ; and hence the intensity of the former force is to that of the latter, as a b to a c. To preserve the index from disturbance by the air, the whole is enclosed in a glass cylinder, at the upper part of which, where the wire is attached, there is an index and a divided circle, which is used to twist the wire, when the measure of a force becomes greater than a whole circumference. The sensibility of this instrument will depend on the dimensions of the suspending wire. Thus, if the length of the wire be doubled, the sensibility will be increased in the same proportion, i. e. only half the force will be required to twist the wire a given number of degrees. If the diameter of the wire be increased, the sensibility is diminished in a great degree ; thus, if there be two wires of the same length, but one twice the thick ness of the other, the latter will require sixteen times more power than the former to twist it through a given number of degrees ; the force of torsion being pro portional to the fourth power of the diameter, and sixteen it will be seen is the fourth power of two. If the wire be increased three times in diameter, the sensibility will be decreased 3 X 3 x 3 X 3 81 times.
Balance of a watch, is that part which, by its motion, regulates and determines the vibrations. The cir cular part is called the rim, and its spindle the verge; there also belong to it two pallets, that play in the fangs of the crown-wheel. In pocket watches, the strong stud in which the lower pivot of the verge plays, and in the middle of which one pivot of the crown-wheel runs, is called the potence ; the wrought piece, which covers the balance, and in which the upper pivot of the balance plays, is the cock ; and the small spring in the new pocket watches, is called the regulator. The motion of a balance, like that of a pendulum, being reciprocating, while the pressure of the wheels is in one direction, it is obvious that some contrivance must be used to accommodate one to the other. When a tooth of the wheel has given the balance a motion in one direction, it must quit it, that it may obey an impulsion in the op posite direction. The balance, or pendulum, thus escaping from the tooth of the wheel, or the tooth escaping from the balance, has given to the general construction the name of scapement. See Hoao