Hydrometer

Hydrometer, device that measures the specific gravity of a liquid (its density relative to that of water). It is based on Archimedes' principle that states that a floating object is buoyed up by a force equal to the weight of the fluid it displaces. The instrument is a long thin glass bulb held upright by a weight at one end, allowed to float in the test liquid. The denser the liquid, the less the hydrometer sinks. The tube is calibrated in such a way that the specific gravity can be read off by comparing the level to which the bulb sinks with a scale along the side of the hydrometer. A form of hydrometer called an acidimeter is used in testing storage batteries by measuring the specific gravity, and therefore the strength, of the acid they contain.

See also: Density.