Under which condition can a viscosimeter measure viscosity effectively?

A viscosimeter measures the viscosity of a fluid by determining how it flows under specific conditions. The correct choice highlights that a stationary condition allows for the most accurate measurement of viscosity. When the fluid is stationary, the viscosimeter can accurately assess the resistance to flow without the influence of external forces that might alter the fluid’s behavior, such as turbulence or shear stress.

Under stationary conditions, the calculation of viscosity becomes straightforward as the only variables involve the fluid's intrinsic properties. This ensures that readings reflect the true viscosity of the material rather than being affected by external movement or flow dynamics.

In contrast, moving conditions can introduce variables that interfere with accurate viscosity readings. Heated conditions could alter the properties of the fluid, leading to variations in viscosity with temperature changes. An open condition might allow for evaporation or changes in pressure, which could affect viscosity measurements as well. Thus, stationary conditions ensure a controlled environment for optimal viscosity measurement.