Jolly balance

The Jollysche spring scale is a measuring instrument for determining the density of a sample. It is named after Philipp von Jolly, who first mentioned her measuring principle on 13 February 1864 in a speech in public. Here, the Archimedes' principle is utilized.

Construction

The scale consists of a frame with two scales, which is suspended from a spring together. The lower shell is located in a liquid, while the upper shell is surrounded by air.

Principle of measurement

The determination of the density with a Jolly 's Libra is a relative measurement method. To determine the density by one must measure the mass and volume of the body. However, the determination of the volume is often very imprecisely. With relative measurement methods to determine the density of an unknown body using the known densities of a comparative substance, which is the liquid in the case of Jolly 's Libra.

Measuring the weight of the arrangement of the elongation of the spring according to the Hooke's law. To the body, the mass of which is to be determined is first set to the upper shell and the measured weight. Then you put the same body on the lower scale, bearing in mind that it is completely covered with liquid. Thereby measuring the weight of which is less than a result of the buoyancy in the liquid.

Calculating the density from the measuring values,

A force is obtained compared to the rest position of the spring elongation. Here, the spring constant. Thus obtained, the density of the body: and, if the buoyancy of the air can be ignored, with

• Density of the liquid
• Density of the ambient air