The idea. Tiny oil drops pick up a few electrons (or lose some) as they are sprayed. Between two horizontal plates there is a uniform electric field E = V/d. A drop has weight mg downwards. If the drop is charged, the field pushes it up or down with a force qE. When the voltage is adjusted so that the drop hangs still, the two forces are equal and opposite:

qE = mg, so q = mg / E = mgd / V

The mass comes from the size of the drop: m = ρ × (4/3)πr³. In the real experiment the radius is found by switching the field off and timing the fall of the drop. The air drags on a small sphere, and a drop falling at a steady speed has weight equal to the drag of Stokes’ law, 6πηrv, so r = √(9ηv / 2ρg). Here the radius is given.

What to do. (1) Press Spray oil. With the voltage off all the drops fall. (2) Click a drop to choose it. (3) Turn the voltage up or down and watch it: if it rises the drop and the top plate have opposite signs, so a positive voltage lifts negative drops and a negative voltage lifts positive drops. (4) Use the coarse and the fine slider until the drop hangs still. The speed reads 0 and the page says Hanging when the forces are balanced to about 0.3%. (5) Press Record this balance. The page works out the charge. (6) Press X-ray burst. The X-rays knock an electron on or off the drop, so its charge changes by one electron (the drop may also have changed by nothing you can see). Balance it again and record. (7) Do this for several drops. Look at the column of charges, and at the column that divides each charge by the smallest one.

What you should find. The charges are not just any value. They are all whole-number multiples of one smallest charge, the charge of one electron, 1.6 × 10⁻¹⁹ C. Millikan used this to show that charge is quantised and to measure e. After three or more good measurements the page estimates e from your data. If all your charges are even multiples, you cannot tell that e is half as big: you need a drop with an odd number of electrons.

Notes on the simulation. The animation runs 100 times faster than real life (a real drop falls about 0.1 mm per second), but the speed shown is the real one. The speeds follow Stokes’ law for a sphere with ρ = 886 kg m⁻³ (oil), η = 1.81 × 10⁻⁵ Pa s (air) and a plate gap of 5.0 cm. The drops are drawn far larger than they are (a real drop is about 2 micrometres across). The correction for the buoyancy of the air, and for the way very small drops slip between air molecules (the Cunningham correction), are left out. A drop moves sideways a little because of collisions with air molecules (Brownian motion). The drops that are made are ones that can be balanced with the voltages available (up to ±15 kV).