A hydrometer measures the relative densities of two liquids. Many hydrometers are calibrated to compare the density of a liquid to the density of water, which provides the specific gravity of the liquid that is tested.
Specific gravity is the ratio of the density of a liquid to the density of water; at a temperature of 4 degrees Celsius, pure water has a specific gravity of 1.
Hydrometers are used by brewers and winemakers to test samples of their products. They measure the liquid's density both before and after the fermenting process, and calculating the difference allows them to determine a product's alcohol percentage.