Experiments · High school and university

Millikan’s oil drop experiment

Adjust the voltage, hold the drop still in the microscope and work out the charge of the electron yourself.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Millikan’s oil drop experiment

Adjust the voltage until the drop stands still in the microscope

View through the microscopeImage turned upright (a real microscope shows it upside down).

q / e

3.02

Drift speed: 0.1 µm/s

hovering

When the drop hovers, this comes out very close to a whole number.

Field E = V/d: 74,333 V/m
Fall speed v₀ (no field): 105.7 µm/s
Radius r from Stokes: 1.001 µm
Weight m·g: 3.591 × 10⁻¹⁴ N
Charge q = m·g / E: 4.831 × 10⁻¹⁹ C

Apparatus

Drag the slider or type a value, then press Enter.

Voltage V

Plate gap d

Drop radius r

Oil density ρ

Measured charges

Each measurement falls into a bin. The lines mark whole multiples of e.

No measurements yet. Make the drop hover, then press “Measure and record”.

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

A drop of radius 1 µm hovers at 446 V. To hold a drop with the same charge but half the radius (0.5 µm), what should the voltage be?

How to measure

Spray a drop with the atomizer; it shows up in the microscope as a bright dot.
Adjust the voltage: lower it if the drop rises, raise it if the drop falls. When it stops, q·E = m·g.
“Measure and record”: q = m·g·d / V is worked out and added to the histogram.
Ionize or spray a new drop and measure again. The charges pile up at multiples of e.

Gold light: the lamp’s beam, which makes the drops shinePurple ray: X-rays, which ionize the airThe numbers come from a single model: g = 9.81 m/s², air viscosity 1.8×10⁻⁵ Pa·s, oil 870 kg/m³ (adjustable). In the model a drop’s charge is always a whole multiple of e = 1.602×10⁻¹⁹ C. The radius comes from the fall speed measured with the field off, using Stokes’ law; a small, seeded stopwatch error is added to that measurement, so q/e does not come out exactly whole. Air buoyancy and the Cunningham correction Millikan applied to small drops are left out. The plate gap and the drop’s position are enlarged in the drawing.

Millikan's oil drop experiment: measuring the charge of the electronRead the article