An instrument used to measure the viscosity of a liquid. Rotational type viscometers include concentric cylinders, and cone and plate instruments. They operate at constant speed, which defines the shear rate. The liquid forms a continuous film between the rotating element and the stationary element. The torque to resist the motion defines the shear rate. The Ostwald viscometer, which is used to measure the viscosity of liquids, consists of a bulb into which the liquid is filled and is allowed to flow under gravity through a capillary tube. It is the time taken for the meniscus to reach a mark on the capillary from which the viscosity can be calculated. A falling-sphere viscometer is based on Stokes’s law, which consists of timing a steel sphere at its terminal velocity under the influence of gravity through the liquid.