Abstract: An apparatus for continuous measurements of the density of variegated liquids, utilizing the angular displacement of the limiting angle at total reflection. Among other purposes the apparatus is useful for measurements of the acid concentration of liquids, such as in a storage battery in a motor vehicle to determine the state of charge of the battery. The apparatus comprises a light rod, in which the end to be immersed in the liquid is provided with a measuring surface and preferably also with a deflecting surface, a light source emitting a bundle of rays into the rod toward the measuring surface at a given divergent angle and a light-sensitive element positioned in the path of rays.