Electrical mobility is also inversely proportional to the Stokes radius a of the ion, which is the effective radius of the moving ion including any molecules of water or other solvent that move with it. This is true because the solvated ion moving at a constant drift velocity s is subject to two equal and opposite forces: an electrical force z e E and a frictional force F drag = f s = ( 6 π π η η a ) s , where f is the frictional coefficient, η η is the solution viscosity. For different ions with the same charge such as Li, Na and K the electrical forces are equal, so that the drift speed and the mobility are inversely proportional to the radius a . In fact, conductivity measurements show that ionic mobility increases from Li to Cs, and therefore that Stokes radius decreases from Li to Cs. This is the opposite of the order of ionic radii for crystals and shows that in solution the smaller ions (Li) are more extensively hydrated than the larger (Cs).