A hydrometer is an instrument used to measure the relative density of a liquid by the method of flotation. It consists of a graduated glass tube with a weight at the bottom to make it float vertically in the liquid. The level at which the surface of the liquid touches the scale on the tube is a measure of the relative density of the liquid. Hydrometers are commonly used in industry and laboratories to measure the density of liquids such as acids, alkalis, and electrolytes.