ASSAY-BALANCE, a very nice balance used in docimastic operations, to determine exactly the weight of minute bodies; see fig. 7. This balance should be made of the best steel, and of the hardest kind; because
that metal is not so easily spoiled with rust as iron; and it is more apt than any other to take a perfect polish, which at the same time prevents the rust.
The structure of the assayer's scale is little different from that of common scales, otherwise than by its nicety and smallness. The longer the beam of it is, the more exact may the weight of a body be found; however, 10 or 12 inches are a sufficient length. Let the thickness of it be so little, that two drams may hardly be hung at either of its extremities without its bending; for the largest weight put upon it seldom exceeds one dram. The whole surface of this beam must be altogether without ornaments, which only increase the weight and gather dust, &c. The beam is suspended in a fork, the two legs of which are steel springs joined at top, but kept together below with a brass pliant clasp, parallel, and two lines and a half distant from each other. This clasp being taken off, and the legs of the fork being stretched out, the axis of the beam may be put into two holes made for that purpose at the ends of the legs, or be taken away from them. Let a very sharp needle be fixed in the head of the fork, standing perpendicularly downwards, if the fork is suspended, and so long, as that it may almost touch the top of the tongue of the beam put into the fork when in equilibrio. This needle is the mark of the equilibrium; and that the artists may be able to observe this, the legs of the fork must be broader in that place, and have an opening two or three lines wide; this fork may be adorned at pleasure, provided the motion of the balance is not hindered by such ornaments: then take two scales made of thin plate of silver, one inch and a half in diameter, hanging on three small silk strings, almost as long as the beam, tied together at top, with a silver hook in form of an S, and hang them to the extremities of the beam: a smaller silver dish or blued steel, somewhat less than one inch in diameter, belongs to each of these scales. You first put into these dishes, with a pair of pincers, the bodies to be weighed, or with a spoon or a small shovel, when they are pounded, and then you put them into the scales; therefore the small dishes must be perfectly equal in weight. We use them, that bodies may be more conveniently put into and taken out of the scales, and that these which are vastly thin may not be bent or soiled, and thence rendered false by wiping.
This balance is suspended on a moveable brass or copper support, which consists of a pedestal, and of a column set upon it about 20 inches high, at the top of which comes out at right angles an arm one inch long. At the extremity of this arm, put a small pulley three lines in diameter, another at the top of the column, and a third near the bottom of it; all which pulleys must turn very easily on their axes. At the distance of one inch and a half below the upper arm, let another arm one inch and a half long come out of the column at right angles, having a hole through it two lines long, a quarter of a line broad, and placed perpendicularly below the pulley of the upper arm, to receive a small plate, one inch and a half long; and of such breadth and thickness, as that it may freely move up and down, and yet not have too much play within the hole. This plate must also have a small hook at each extremity.
And as such a balance will hardly stand still in the
open air, and becomes false when spoiled with dust; Balance, it must be put, together with its support, into a small case as represented in fig. 7. having glasses, a, a, a, at top, and all round it, that you may see what is within.
Manner of using the assay-BALANCES.—Pass a silk string over the three pulleys of the support, and tie it at its upper extremity to the small hook introduced into the hole of the inferior arm; then put the support in the middle of the small case, and pass the other extremity of the silk string below, through a hole bored in the middle of the lower part of the frame, containing the window in the forepart of the case, and fasten it to a small weight of a cubic form. Suspend the fork of the balance on the inferior hook of the plate. By this means if you move backwards and forwards the weight fastened to the string, placed upon the top of the drawer jutting out beyond the fore-part of the case, the balance within is either lifted up or let down. But you must put the bodies to be weighed, and the weights themselves, in the small silver dishes; and these, when loaded, into the scales, thro' the side-windows, which must be opened for that purpose. When any thing is to be added to or taken out of them, you do it with the small pincers; or, if it is powder, with the small shovel or spoon: but you must let the balance down every time any thing is to be added or taken away, that the scales may rest upon the bottom of the case; and shut the windows before the balance is lifted up again, especially if the air is not perfectly calm.