Millikan’s oil-drop experiment proved that electric charge is quantized.
Millikan sprayed small oil droplets into a chamber.
Many of the droplets either gained or lost a few electrons through friction with the nozzle.
He used an electric field to suspend these tiny charged drops between two electric plates and measured the charges on the drops.
How did this work?
As the droplets fell through a hole into the viewing chamber, an X-ray source was used to ionize the air molecules between two plates, knocking out their electrons.
Many uncharged oil droplets then acquired these electrons, making them negatively charged.
When the plates were uncharged, the only forces acting on the droplets were the gravitational force downwards and the drag force upwards against their motion.
Since the gravitational force (in this case) was higher than the drag force, the resultant force was downwards.
However, when he gave the top plate a positive charge, the charged oil droplets were affected by the electrical field.
Another force acted on them — the electrical force.
The drops with a positive charge were repelled downwards, but the negatively charged drops were attracted upwards.
He was able to adjust the voltage so that the upward force just balanced the downward gravitational force.
Since the forces were equal, the drops were suspended.
Millikan found that the charges on the drops were all multiples of
He proposed that the elementary units (quanta) of both positive and negative charge were the same, namely