WEIGHING MACHINES, mechanical de %ices for ascertaining the weight of objects. It is to be understood, however, that the ele ment thus determined is a relative value, and is entirely independent of the magnitude of the force of gravity or the tendency of a body to fall. It is measured at sea-level and at the mean diameter of the earth. For example, the force of gravity decreases with elevation, but a commercial unit of weight such as a 'pound* is the same at the top as it is at the foot of a mountain. In the usual method of determining weights of this character, a "unit of weight is lust adopted, and then a "set of weights) procured. each of which represents a certain predetermined number of the units. With these, by the aid of a machine so constructed as to be capable of establishing an equilibrium between any number of the weights and the article being weighed, the ratio of the weight of the article to that of the adopted unit is determined. Such machines may be conven iently arranged in three general classes `equal armed' balances, 'unequal armed' bal ances. and Spring' balances.
Equal armed balances are represented by the various forms of 'scale-beams,' in which the scale-pans hang below the beam, and the "counter-scales* in which the scale-pans are above the beam, both forms operating by ad justment of equilibrium between a known and an unknown weight. Unequal armed balances are represented by those consisting of a single steelyard, and those formed by combinations of unequal armed levers and steelyards, such as platform scales and weighbridges. in which a small known weight at one end of a beam counterbalances a heavy unknown weight at the other end. Spring balances operate on the prin ciple that, when a helix is subjected to a ten sion, the amount of elongation increases pro portionally with the increase in the force of tension In the simplest form, it consists of a helical spring of hard steel of a high elastic limit. As commonly used it is suspended from a fixed point by a hook at its upper end, while its lower end is bent into a hook, which is cro-sed by another hook to which the article to be weighed is attached. An index attached to the spring and moving along the face of a graduated plate indicates the amount of ten s:on in terms of the adopted unit of weight. The contrivance is so arranged that the axis of the spiral is always a plumb line, under tension or at rest, and is capable of weighing only an amount within the elastic limit of the spring. The capacity of such balances is gen erally small, and they are extensively used in shops, and for other purposes where a high degree of accuracy such as may be obtained by lever balances is not required. On the other hand, the various forms of dynamometers em plus ed for registering the pull of locomotives are made with enormous capacities. In this connection the 'torsion balance' may be con s eniently mentioned, although in reality it is a machine generally used for the purpose of measuring horizontal forces, rather than weights as here understood. In its simplest form it con
sists of a beam, supported by a wire stretched horizontally which passes through its centre of gravity. When twisted by the application of a weight to the beam, the elasticity of the wire acts the part played by the weight of the beam in an ordinary balance. For measuring hori zontal deflections' caused by electric, magnetic, and other forces, the wire is stretched verti call y.
For all forms of weighing machines of the equal armed type with small capacities, such as counter-scales, chemical and assay balances, etc., Set BALANCE.
The majority of the machines of large ca pacig known as •platform-scales,' used for weighing heavy loads such as carts loaded with grain, hay, coal, building materials, etc., oper ate on the principle of the lever, and belong to the class designated as unequal-armed balances. They are sometimes arranged as •decimal bal ances,' the ratio between the small known weight on the weighing beam and the unknown weight on the platform or bridge being indi cated in multiples of 10. Such a balance con sists of a lever (ab), suspended at (h), and connected by the rod (o), with two levers (r) and (s), hinged at (t) and (u), respectively. A platform (p), resting by means of standards upon the levers (r) and (s), accommodates the load which is counterbalanced on the long arm of the weighing beam or lever (ab), by a sliding weight— (w). When a load is placed upon the platform (p), the levers (r) and (s), are depressed, and exert a downward pull on the short arm of the weighing beam through the rod (o), with a consequent lifting of the long arm. This pull is counterbalanced by sliding the weight (w) along the long arm until it is brought into a horizontal position. The weighing beans being graduated with an adopted unit of weight and fractions thereof (usually pounds), the weight of the load on the platform is ascertained by a simple direct read ing of the graduated scale. In the smaller scales, the weight (w) equals one pound, and counterbalances a load of 100 pounds upon the platform, but it is evident, that by varying the number of levers and the relative lengths of their arms, any desired ratio between the load and the counter weight may he readily obtained. In scales used for weighing loaded wagons and cars, the ratio varies from one to 500, or one to 1.000. Cattle scales are capable of weighing from 10 to 200 head of cattle at a single opera tion, and the capacity of grain scales ranges upward of 500 bushels, while the large iron and steel manufacturing plants and gun shops employ machines with capacities exceeding 100, 000 pounds. The machine at Watervliet Ar senal, Watervliet. N. Y., used for weighing guns, has a capacity of 300,000 pounds.