If we had performed the experiment with the test-tube and lead shot just described with several liquids of different densities, we should have found that the tube sank to a different level in each of the liquids. In every case, however, the total mass of liquid
displaced (equal to volume x density) would have been equal to the mass of tube and shot, which is constant. This is the principle of an instrument called a hydrometer, used for rapid and measurement of the relative density of liquids.
The modern form of hydrometer shown in Fig. 12.8 (a) differs scarcely if at all from those made by Robert Boyle in the seventeenth century. The lower bulb is weighted with mercury or lead shot to keep it upright, and the upper stem, graduated to read the relative density of the liquid, is made thin to give the instrument a greater sensitivity. Such hydrometers are usually made in sets of four or more, each covering a different range.
In addition, others are obtainable for special purposes. One, called a hectometer, has a range of 1.015-1.045 and is used for testing milk. Another, enclosed in a glass tube fitted with a rubber bulb, is used for measuring the relative density of battery acid (Fig. 12.7 (b)). On squeezing the bulb and then releasing it, it expands causing the air pressure inside to decrease. The greater atmospheric pressure outside pushes acid up into the glass tube, and the density can then be read on the floating hydrometer. The acid in a fully charged cell should have a relative density of 1.25- 1.30. A reading of less than 1.15 indicates that recharging is necessary.