JOLLY BALANCE is a device for determining the spe cific gravity (relative density) of solids and liquids. Its operation is based on (a) Archimedes's principle; i.e., that a body im mersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced, and (b), on Hooke's law; i.e., that in elastic bodies and within certain limits the strain is proportional to stress, or that displacement is proportional to displacing force. In its usual form the Jolly balance consists of a long, delicate, helical spring suspended by one end in front of a uniformly graduated scale. To the lower end of the spring is attached a weight pan, and immediately below that a small wire basket. At some convenient point, usually just above the weight pan, an index is provided which can be read with reference to the scale, which is usually graduated on the surface of a mirror to avoid parallax in reading the position of the index. A movable plat form for supporting a small vessel of water is provided which can be clamped at any desired height along the vertical scale and immediately below the wire basket attached to the lower end of the spring. The point of suspension of the upper end of the spring is also usually movable vertically, as a means of increasing the range of the balance and as an aid in adjusting the index to a convenient zero setting.
In determining the specific gravity of a solid a zero reading of the position of the index is taken with the wire basket im mersed in water up to a definite point on its suspension wire, care being taken to see that the point of suspension of the helical spring is such as to permit the necessary extension of the spring during subsequent steps of the determination. Let this read
ing be W,. Next, place in the weight pan the specimen, the specific gravity of which is to be determined, lower the movable platform carrying the vessel of water until the water surface reaches the same point on the suspension wire as before, and again read the index. Let W2 represent this reading. W2—W1, which represents the extension of the spring, may be taken as the weight of the specimen in air. Then place the specimen in the wire basket, immersed in water, and again adjust the height of the movable platform until the water surface is at the original height on the suspension wire above the basket, and again read the posi tion of the index. Let this reading be W3. W2— W3, which repre sents the difference in the extension of the spring when the specimen is immersed in air and in water, respectively, may be taken as the loss of weight in water. Then specific gravity The specific gravity so obtained is the apparent specific gravity in air, in terms of water at the temperature of the determination. In order to determine the specific gravity of a liquid, the same procedure is followed except that the "loss of weight in water" and the "loss of weight in the unknown liquid" are determined with any convenient specimen of a solid not affected by either liquid. The specific gravity of the unknown liquid is the ratio of the loss of weight in the unknown liquid to the loss of weight in water. (See also BALANCE.) (H. W. BEA.)