Millikan's Oil Drop Experiment

In 1909, Robert Millikan set out to measure the charge on a single electron. Millikan knew that oil drops sprayed from an atomizer often picked up a static charge from the atomizer. in other words, he knew that oil drops often stole electrons from the nozzle of the atomizer to end up with a negative charge. (Because this is inconsistent, ionizing radiation is now used to ensure a charge on the oil drops.)

Because negative charges are attracted to positive charges and repelled by negative ones, Millikan contrived to put his negatively charged oil drops between to charged metal plates (the bottom one negative to repel the drop upwards and the top one positive to pull the drop upwards). In this way he could cause the drop to float in midair (the force up from the two plates balancing gravity).

This is the apparatus that Millikan devised.

 

 

 

 

 

 

 

 

 

 

 

The procedure followed was this:

1. Spraying the oil from the atomizer would release a small cloud of nearly invisible drops of oil, many with a negative charge.
2. At least one drop would fall (due to gravity) through the hole in the top plate
3. Millikan would focus on one particular drop using the telescope
4. The charged plates would be turned on creating an upward force on the drop
5. The voltage on the plates would be adjusted until the drop was suspended in midair.

When the drop was suspended, Millikan knew that the force up and the force down must be equal.

More on Millikan and his math