JOLLY BALANCE. A piece of physical ap paratus used in del'r•niiuing tlre specific gravity of small objects. especially minerals. It corr sists of a vertical standard from the tipper end of which is suspended a spiral spring of fine wire. To the lower end of this spring are at tac•Iled small pans, in each of whielt cau sed tively is placed the object whose specific gravity is to he determined. A scale—usually a mirror on which equal divisions are etelad —is placed be hind the spring, and the position of some fixed point, such as a bead, is noted on the scale. The object i then placed in the upper pan, and the spring being extended, the fixed point will he opposite some lower division on the scale. The amount that the spring is extended is noted, and after rcmaving the object weights enough to pro duce an equal extension are put in its place. The object is then placed in the lower pan, which is inuuersed in a vessel of water, and as the buoy ant effort of the latter acts on the object the amontt the spring is extended is diminished and the fixed point will be opposite some higher divi sion on the scale, which, of course, is noted.
Weights are them added to stretch the spring the same amount. The difference between the weights required to stretch the spring from its zero point to where it rests when the body is weighed in the air, and the amount required to bring it to the position it assumes when the pan and body are in the water, divided into the total weight, gives the specific ity.
The usual arrangement of the apparatus is to have two pans, the lower of which is kept sub merged in the water so that the conditions remain constant whether the body is placed in the upper or lower pan. The etched mirror usually employed forms an admirable scale, inas much as it enables the observer to avoid parallax and deter mine accurately the position of the fixed point, as when the bead or other point on the spring is seen with the eye, re flected together in the mirror, it implies that the eye, the fixed point. and the settle division are all in a straight line perpen dicular to the surface of the mirror. In an accurate deter mination correction should be made for the temperature of the water, as explained under SPECIFIC GRAVITY, and the wire which carries the lower pan should be as fine as possible to eliminate the effects of capil larity between the water and the wire.
The Jolly balance is used mostly for rapid determination of specific gravity of such objects as mineralogical specimens, and does not admit of as great precision as an analyt ical balance arranged for specifie-gravity deter minations. The spring must be suited to the weights of the objects which are to be used, so that the extensions will not be produced over too long or too short a range of scale. In the latter case a small error of observation will seriously affect the result. See SPECIFIC GRAVITY.