Stokes formula described very well the homogeneous balls moving in a viscid, continuous medium. But Milikan came to the conclusion that, although the droplets of oil could be assumed as homogenous, still the medium in which the move can't be told continuous. The main path between collisions of droplets and atoms of the air was equal 10-5 centimeter. That distance called 1 is much shorter than the radius a of the drop (a is equal 10-4). In that case the velocity of the drop in the gravity field is equal the product of the velocity given by the Stokes' formula and some factor of correction that is the function 1/a. Milikan assumed that the factor of correction could be expanded into the infinite power series:
(1)
where v is the velocity from the Stokes' formula, and the coefficients A,B etc. could be determined experimentally. Milikan noticed that it is enough to leave in the formula only the A coefficient because the rest of them didn't influence v0 much:
(2)
The uncorrected radius of the drop is from Stokes formula:
(3)
So after the v0and the v evaluation there is:
(4)
and after the transformation:
(5)
