What was the big surprise in Millikan's oil drop experiment?
There was no big surprise in Millikan's oil drop experiment.
The big surprise came in his earlier experiments. Here's the story.
In 1896, J.J. Thomson had shown that all cathode rays have a negative charge and the same charge-to-mass ratio.
Thomson tried to measure the electronic charge. He measured how fast a cloud of water droplets fell in an electric field.
Thomson assumed that the smallest droplets, at the top of the cloud, contained single charges. But the top of a cloud is quite fuzzy, and the droplets evaporated rapidly. The experiments gave only a crude value for the electronic charge.
In 1903, Charles Wilson used a 2000 V battery to charge two metal plates. He studied the rate of fall of the top of the cloud both under gravity and when the voltage as well as gravity was driving the drops downward.
In 1908, Millikan repeated Wilson's experiments. He used a 4000V battery to make the drops fall faster and reduce the rate of evaporation. But that reduced the viewing time for a drop.
Millikan then decided to reverse the electric field and try to hold the droplets stationary. He used a massive 10 000 V battery.
To his surprise, application of the powerful ﬁeld dispersed the cloud immediately. Left in view were a few droplets that had just the right charge for the electric force to balance the effect of gravity.
Millikan could hold individual drops suspended for as long as 60 s.
This was his big breakthrough.
Millikan then replaced the water with oil to reduce the rate of evaporation.
The rest is history.