A type of flow meter used for the measurement of flow of gases and liquids by virtue of the elevation of a solid float within a vertical, tapered (p. 400) tube. The tube is usually made of glass while the float is made of metal, ceramic, or plastic with a float density greater than that of the fluid. The float is usually bomb-shaped having a cylindrical body with a coned bottom and short top piece, which is slightly larger in diameter and often grooved to encourage the float to spin, thereby improving stability due to a gyroscopic effect. The upwards flow of the fluid causes the float to rise and reach an equilibrium elevation. The flow is read from a calibrated scale on the tube. There is a roughly linear variation of mass flow rate with float position in the tube. In practice, a scale is marked on the tube. Calibration curves are used for a particular fluid, temperature, and float. Unlike the venturi or orifice type flow meters, it operates with a fixed pressure drop across the float and has a variable area around the float. The Rotameter is a registered name for a type of variable area flow meter.