We can make use of these known values to measure the values of ,  and  for unknown surfaces.
To do this, we take 3 liquids, with at least two of them polar, and put a drop of each of these liquids on the surface. These liquids are called probe liquids. We measure the contact angle of these drops with the surface and then use Young’s equation. The device used to measure the contact angle is called a goniometer.
We now have 3 unknowns and 3 equations which can be solved to obtain ,   and  for the surface. But there is a problem with this procedure.
It has been found that it is the advancing rather than receding equilibrium contact angle that should be measured. This is because when receding equilibrium contact angle is measured, the surface has already been contaminated by the probing liquid.