A direct-reading instrument used for measuring the density or relative density of a liquid is called a hydrometer. A hydrometer is based on the principle of floatation, i.e., the weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer. A hydrometer is a glass instrument. It consists of a cylindrical bulb having a spherical bulb at its lower end and a long stem (narrow tube) at its upper end. The lower spherical bulb is partially filled with lead shots. This helps the hydrometer to float (or stand) vertically in liquids. The stem has markings so that the relative density of a liquid can be read off directly. The hydrometer is placed in the liquid and taken in a glass jar. The reading against the level of liquid gives the relative density of the liquid.