The charge of an electron was first measured by the American physicist Robert Millikan during (1909-1913). In his experiment, oil was sprayed in very fine drops (around 10 –4 mm in diameter) into the space between two parallel horizontal plates separated by a distance d. A potential difference V AB is maintained between the parallel plates, causing a downward electric field between them. Some of the oil drops acquire a negative charge because of frictional effects or because of ionization of the surrounding air by X-rays or radioactivity. The drops are observed through a microscope. ρ is density of oil and r is radius of the drop under investigation.

(i) The charge so that oil drops remain at rest is –