A hydrometer is an instrument used to measure the specific gravity or relative density of liquids, i.e. the ratio of the density of the liquid to the density of water. Consisting of a cylindrical stem of glass with a weighted bulb at one end, the hydrometer is designed to float upright. The liquid to be tested is poured into a narrow container, slowly to avoid bubbles, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted.
