Quantization of Electric Charge
Also known as: Millikan oil-drop result · Charge quantization
Charge doesn't come in arbitrary amounts — it comes in whole-number multiples of one indivisible unit e. Millikan proved it by suspending tiny charged oil droplets between charged plates: turn up the voltage until an electron's electric pull exactly balances gravity. Every drop's charge came out as an integer times the same tiny number, 1.6×10⁻¹⁹ C. You never find half an electron's worth. Charge is granular, like money in indivisible pennies.
An oil drop hovers when the electric force balances gravity; changing the charge count unbalances it.
Equivalent forms
One dangling oil drop, balanced against gravity, revealed that the universe counts charge in integers — and gave the first precise value of e.
Where it holds
Dimensional analysis
Millikan and Fletcher sprayed oil into a chamber and watched individual droplets rise and fall between charged plates through a microscope. By balancing gravity against the electric force they measured each drop's charge and found them all multiples of one value. The 1913 result gave e = 1.592×10⁻¹⁹ C — slightly low because Millikan used a wrong air-viscosity value — and won him the 1923 Nobel Prize.
- Defining and realizing the SI ampere via fixed e
- Single-electron transistors and charge sensing
- Calibration of electrometers
- Quarks having e/3 does not violate quantization of free charge — quarks are confined
- The drop's mass is not weighed directly; it is inferred from its fall speed
- Millikan's slightly-off value came from an erroneous viscosity, not from cherry-picking data
What if…
Millikan's drops would show a smooth spread of values instead of clustering at integer multiples — and atoms could not be electrically neutral in exact integer ratios.
It gains or loses whole electrons, so its charge jumps in discrete steps of e — exactly what Millikan watched happen.
Charges on a balanced drop
- m:
- 3.3e-15 kg
- E:
- 2.0e5 V/m