The Millikan oil-drop experiment and the elementary charge

Millikan's apparatus was simple in outline: spray a fine mist of oil into a chamber between two horizontal capacitor plates, let a few droplets pick up stray charge as they form, and watch one through a microscope. With no field applied, gravity alone pulls the droplet down, and its terminal velocity through air reveals its mass. Switch on a field between the plates and adjust its strength until the droplet hangs perfectly still — at that exact point, the electric force pushing up exactly cancels gravity pulling down, qE = W, so the droplet's charge can be read straight off: q = W/E. The drop isn't motionless because nothing is happening to it; it's motionless because two real forces have been tuned into exact balance.

Millikan did this for droplet after droplet, and every single measured charge — no matter which droplet, no matter how it happened to pick up its charge in the spray — turned out to be a whole-number multiple of the same smallest value, roughly 1.6 × 10⁻¹⁹ coulombs. That result was the real discovery, not just a measurement of one number: electric charge isn't a continuous quantity that can take any value at all, the way mass or length seem to. It comes in indivisible units, later understood as the charge of a single electron (or proton), and nothing in between is ever observed.

It's worth being precise about the direction of the physics inside the balance condition. A more heavily charged droplet feels a bigger push from the same field, so it takes a smaller field to balance the same weight — not a bigger one. Millikan exploited exactly this: by slightly ionising the air around a droplet mid-experiment and watching its charge (and hence its balancing field) jump by a fixed amount each time, he could see the quantization directly, one discrete step at a time.