Hydrometer

Hydrometer, in chemistry, graduated glass or metal instrument used to measure either the specific gravity or density of a liquid. It is based on the hydrostatic principle of the Greek mathematician and inventor Archimedes, which states that the weight loss of a body in a liquid equals the weight of the liquid displaced. Most hydrometers are enclosed in glass tubes fitted with rubber bulbs for drawing up the solution to be measured. The sealed instrument itself floats and has a bulblike bottom weighted with lead or mercury. When immersed, the graduated stem rises vertically to give a scale reading. Hydrometers must be calibrated according to the type of liquid to be tested and at a standard temperature, usually 4° C (39.2° F), or 20° C (68° F). Various types of hydrometers measure density or purity in storage batteries, ship boilers, soil, and milk.