Stokes Falling Body Viscometer

 

Stokes considered the viscous force (F) which is exerted on a sphere of radius (a) when moving at a uniform velocity (v) through a liquid having a viscosity () and derived the following equation,

 

 

Thus for a sphere falling under gravity then,

 

 

Where () is the density of the sphere and () is the density of the liquid.

 

In practice the viscosity of the liquid is determine measuring the transit time for the sphere to traverse the center third of the total length of fall. The expression for the viscosity including a correction for the finite length of liquid is as follows,

 

 

Where (R) is the radius of the tube and (h) is its length.