A hydrometer is an instrument used to measure the specific gravity (relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely.