hydrometer
A hydrometer is an instrument used to measure the the ratio of the density of the liquid to the density of water, or the relative density of liquids (referred to as specific gravity).ย 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 jar (the test jar), 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 read against a paper scale that is inserted inside of the stem. ย The reading may be in a scale of specific gravity or brix, depending on the purpose.