STEELYARD. A balance by which the weights of bodies are determined by means of a single standard weight. In the Roman steelyard, or staters, the lever was so constructed that the centre of gravity was brought immediately over the point of support ; and the system be ing accordingly balanced upon its fulcrum F, the effect of the weight of the lever was neutralized. The longer arm was then divided into parts, each equal to the shorter arm, and those again equally subdivided. Suppose now the length of the shorter arm, or the distance F B to be one inch, and the constant weight P to be one pound ; then if P be placed at the distance of five inches from F, it will make equilibrium with a load of five pounds suspended from B; for, from the property of the lever, when the equi librium is established the weight P is to the load at B as the distance of B from F is to the distance of P from F. What ever proportion, therefore, F P has to F B, the same proportion has the weight suspended from B to the constant weight r.
The steelyard in common use is con structed somewhat differently, the beam being seldom made so as to balance itself on the fulcrum Fs hut the error that would arise on this account is compen sated by beginning the divisions at that point where the weight P being placed, the equilibrium is established. If, there fore, when P is removed the longer arm preponderates, the divisions commence from a point between F and B. For the purpose of increasing the range, there are also in general two fulcra, from either of which the beam may be suspended, and two corresponding scales of division are marked on opposite sides of the longer arm.
For weighing heavy loads the steel yard is a convenient instrument ; but for smaller weights it is susceptible of less accuracy than the common balance. It should be constructed so that the point of support F, and thepoint of suspen sion at B, may be in the same straight line with the divisions of the beam.