By Andrew Venzie
In the senior seminar course for physics majors, Taylor Worthington was given the opportunity to recreate Millikan’s Oil Drop Experiment, first conducted in 1909 by Robert A. Millikan and Harvey Fletcher. The experiment was performed in order to measure the charge of an individual electron, a subatomic particle with a negative charge and the primary carrier of electricity.
“To measure this charge, mineral oil droplets are placed between two charged metal plates,” says Worthington. “These metal plates form what is known as a capacitor and imposes an electric field in the space between the plates. As charged droplets of oil enter this space, they will either begin to move upward or begin to move downward faster depending on the orientation of this electric field and charge on the droplet.”
By measuring the speed of these oil droplets as they rise and the speed at which they fall, one can determine the charge of an electron. Upon completing this experiment, the results should support the theoretical value for the charge of an electron.
In the photo to the right, the full setup of this equipment is shown. In the top right, the large black box is the current supply that can be manually controlled. Right below that is the bottle of mineral oil and the spray bottle used to inject the oil droplets into the system.
Towards the left is the apparatus itself. The gold cylindrical plates make up the capacitor where the oil droplets will be placed. Just below that is a viewing scope used to see the droplets themselves rising and falling. On the lens of the viewing scope, there are tick marks that are used to measure the distance that the oil drop falls in a given time period.
With an ordinary stopwatch, you can measure the duration of the fall. With the stopwatch measuring the duration and the tick marks measuring the distance traveled during that duration, you can find the speed by dividing the distance by the time.
This experiment is only one of at least a half dozen that the physics department has purchased this summer. The list includes experiments such as the Cavendish Experiment, the Frank-Hertz Experiment, the Compton Effect, Faraday Effect and Michelson-Morley Experiment, among others.