Droplette

Meet Droplette, a tiny oil drop who floats between two charged plates.

In 1909, Robert Millikan and Harvey Fletcher sprayed fine oil drops between two horizontal metal plates. Some drops picked up electric charge. By adjusting the voltage, they could make the upward electric force on a drop exactly balance its weight, so it hung still.

Then qE = mg, so the charge is q = mg / E. A drop of mass 1.0 × 10⁻¹⁵ kg weighs about 9.81 × 10⁻¹⁵ N. If it carries three elementary charges, it balances when E ≈ 9.81 × 10⁻¹⁵ / (3 × 1.602 × 10⁻¹⁹) ≈ 2.0 × 10⁴ V/m.

After measuring many drops, Millikan found that every charge was a whole-number multiple of one value, about 1.6 × 10⁻¹⁹ C. Charge comes in chunks, and that chunk is the charge of a single electron.

Millikan received the 1923 Nobel Prize in Physics for his work on the elementary charge and on the photoelectric effect. The value is now fixed exactly in the SI.