Robert Millikan (1909)

Discovering the Charge of an Electron

Before 1909 scientists had theorized of another particle inside the atom was present but were unsure of what it was because they could not determine the mass of an atom. Robert Millikan conducted an experiment using drops of oil, and two metal plates that were subjected to a downward force of gravity and an upward electrical field. Because the plates and oil were charged, they did not drop from their suspension, thus confirming that electron do have discrete and measurable charge. Thus, Millikan was able to calculate the mass of an electron, proving its existence.