Weighing. Machines, contrivances for ascertaining the measure of gravity (or weight) of different bodies. The simplest and most accurate form of weighing machine is the common balance, a lever of the first kind with equal arms. This may be made sensitive to within less than 1/6000 of a grain. (See Balance, vol. ii., p. 234.) Weighing machines in which a small weight may be made to counterpoise a large one may be in the form of the steelyard, which is a lever of the first kind with the two arms in varying proportions, one arm being sometimes 300 times as long as the other, so that a two-pound weight would be capable of balancing and therefore measuring the weight of a body of 600 lbs. Compound levers are usually employed for heavy goods, arranged in the form of what are called platform scales. They are used in the weighlocks of canals for weighing boats (see Canal, vol. iii., p. 681), and in market places for weighing car and dray loads. The arrangement is shown in the article Mechanics, vol. xi., p. 327. Machines of this kind are constructed in works of great extent at St. Johnsbury, Vt., in New York, and various other places, and of all sizes, from those adapted for the use of families, grocers, and druggists, that may be placed upon a table, up to those of a capacity of 200 to 500 tons.
The only other kind of what may strictly be called weighing machines is the spring balance. (See Balance.) Of this there are various forms, in which a spiral spring by coiling or uncoiling measures the force of gravity of the body weighed by means of a revolving index. When the spring is made of well tempered steel, the spring balance answers#for most purposes where great accuracy is not required; but the balance with equal arms is the only one which can be used in weighing very small quantities. A hydrometer may be used to weigh solid bodies and estimate their specific gravities, as well as the specific gravities of liquids; but it is not considered in the ordinary sense as a weighing machine. (See Htdrometer).