He realized that trying to determine the charge on individual droplets might work better than measuring charge on whole clouds of water. In he began the experiments, but soon found that droplets of water evaporated too quickly for accurate measurement. He asked his graduate student, Harvey Fletcher, to figure out how to do the experiment using some substance that evaporated more slowly.
Fletcher quickly found that he could use droplets of oil, produced with a simple perfume atomizer. The oil droplets are injected into an air-filled chamber and pick up charge from the ionized air. The drops then fall or rise under the combined influence of gravity, viscosity of the air, and an electric field, which the experimenter can adjust.
The experimenter could watch the drops through a specially designed telescope, and time how fast a drop falls or rises. After repeatedly timing the rise and fall of a drop, Millikan could calculate the charge on the drop. The field was turned on and, if it was large enough, some of the drops the charged ones would start to rise. This is because the upwards electric force, F E , is greater for them than the downwards gravitational force, g.
A charged rubber rod can pick up bits of paper in the same way. A likely looking drop was selected and kept in the middle of the field of view by alternately switching off the voltage until all the other drops fell. The experiment was continued with this single drop. At this constant rate, the force of gravity on the drop and the force of the electric field on the drop are equal:. Q is the charge of an electron, E is the electric field, m is mass of the droplet, and g is gravity.
One can see how Millikan calculated the charge of an electron. Millikan found that all drops had charges which were multiples of 1. Experimenting with cathode rays in , J. George FitzGerald and Walter Kaufmann found similar results. In , Millikan won the Nobel Prize in physics in part because of this experiment. The experiment has since been repeated by generations of physics students—although it is rather expensive and difficult to do properly.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This meant that it was electrons carrying this unit charge. An atomizer sprayed a fine mist of oil droplets into the chamber. Some of these tiny droplets fell through a hole in the upper floor.
Millikan first let them fall until they reached terminal velocity. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop.
Next, Millikan applied a charge to the falling drops by illuminating the bottom chamber with x-rays. This caused the air to become ionized, and electrons to attach themselves to the oil drops. By attaching a battery to the plates above and below this bottom chamber, he was able to apply an electric voltage.
The electric field produced in the bottom chamber by this voltage would act on the charged oil drops; if the voltage was just right, the electromagnetic force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air. Now you try it.
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