Elementary charge

Millikan and Fletcher measured it in 1909 by balancing charged oil droplets between capacitor plates and watching the charge jump in steps, never fractions. Their answer was about 0.6 % low because Millikan used a bad value for the viscosity of air — and the published value crept upward for years afterwards as experimenters found reasons not to stray too far from his number, a case of confirmation bias Feynman liked to tell as a cautionary tale.

Since 2019 e is exact by definition and the ampere is derived from it: one ampere is a flow of 1/(1.602 176 634 × 10⁻¹⁹) elementary charges per second. Free charge is always an integer multiple of e — quarks carry ±e/3 and ±2e/3, but confinement means you never meet one alone. The electron's charge is −e; a coulomb is 6.24 × 10¹⁸ elementary charges, which is why a 2000 mAh phone battery has shuffled about 4.5 × 10²² electrons by the time it is flat.