Dynamic Viscosity measure by Falling Sphere Method

Principle : This method is based on Stoke's law : If a small sphere is moving with constant velocity U through a viscous fluid μ it will experience a drag force

F = 3πμUd

d = diameter of the sphere

U = Velocity of the sphere

Formula to calculate viscosity through this method is 

$$\mu =\frac{d^{2}g[{\rho }_s-{\rho }_f]}{18U}$$

ρs  = density of sphere ball
​  ρ s​  

ρf  = density of fluid​  ρ s​ 

Working: In this method a long vertical tank is taken which is filled with liquid whose viscosity need to be calculated.To make sure temperature of the tank remains constant it is surrounded by another to keep the temperature of tank constant.

Now we put spherical ball of diameter d .When ball fall into the liquid it first accelerates and then fall at constant speed.The constant speed is calculated by special equipment.Then with the known values of d,density of the fluid , density of the sphere we can caculate the viscosity of the fluid

$\mu$μ