Millikan's oil-drop experiment
Oil-drop experiment was the first direct and compelling measurement of the electric charge of a single electron. It was performed originally in 1909 by the American physicist Robert A. Millikan. Using a perfume atomizer, he sprayed tiny drops of oil into a transparent chamber. At the top and bottom were metal plates hooked to a battery, making one positive (red in animation) and the other negative (blue in animation). Since each droplet picked up a slight charge of static electricity as it traveled through the air, the speed of its motion could be controlled by altering the voltage on the plates. When the space between the metal plates is ionized by radiation (e.g., X rays), electrons from the air attach themselves to oil droplets, causing them to acquire a negative charge. Millikan observed one drop after another, varying the voltage and noting the effect. After many repetitions he concluded that charge could only assume certain fixed values. The smallest of these portions was none other than the charge of a single electron.

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Turn voltage on/off when you hear warning sound (if you did not turn on "auto" mode)

The oil will drop down due to gravity and reach terminal velocity (v1).
Please turn on voltage when oil almost reach the bottom of the plate (click the "Voltage on/off"  checkbox at the lower left corner of the applet)
The oil drop will move up and reach another terminal velocity (v2).
From v1,v2 and Volateg you can calculate the charge on the oil drop (may be 1-5 electron --assigned randomly)

You can change the voltage between parallel plate, and you will get different terminal velocity v2, and voltage U. But the calculated value for the charge will be the same).
Click "change Q" will change the number of electron on the oil drop.
Click "new drop" will create another oil drop (different size and different charge).
If you click auto, the simulation will turn on/off voltage automatically.

Please check out [url=] MILLIKAN OIL-DROP EXPERIMENT[/url] or [url=] Millikan's original paper[/url] for more detail information.