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The oil drop experiment performed by Robert Millikan in 1909 enables us to calculate the elementary charge.

The following forces have effect on the droplet:

  1. $F_\mathrm{G} = m g = \frac{4}{3} \, \pi r^3 \rho_\mathrm{Oil} g$
  2. $F_\mathrm{A} = \frac{4}{3} \, \pi r^3 \rho_\mathrm{Air} g$
  3. $F_\mathrm{E} = q E = \frac{qU}{d}$
  4. $F_\mathrm{R} = 6\,\pi\eta r v$

However, which steps of calculation are necessary to finally calculate the elementary electric charge (charge of the electron) or at least a multiple of it?

$$q = \frac{9 \pi d}{2 U} \sqrt{\frac{\eta^3 (v_1 - v_2)}{\rho^* g}} (v_1 + v_2)$$

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closed as off topic by David Z Sep 5 '11 at 21:14

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Did you check the wikipedia ? – anna v Jun 19 '11 at 17:04