Viscometers are used to measure the viscosity (the resistance to internal flow) of a fluid under defined flow conditions. The different types of viscometry instruments have either the fluid stationary and let the object pass through the fluid or vice-versa. Hence, these instruments measure the drag caused by the interaction between the fluid and the object surface. In one version, the time taken for a given volume of fluid to flow through an opening is recorded. In the capillary tube viscometer, the pressure needed to force the fluid to flow at a specified rate through a narrow tube is measured. Other types depend on measurements of the time taken for a sphere to fall through the fluid, or of the force needed to rotate the inner cylinder of a coaxial pair (the space between the two cylinders being filled with the fluid under test), or of the rate at which oscillations of a disk vibrating in the fluid die out.