Millikan Movies

Since Robert Millikan discovered the quantization of electric charge and measured its fundamental value over 90 years ago, his oil-drop experiment has become essential in physics laboratory classes at both the high school and college level. As physics instructors, however, many of us have used the traditional setup and experienced the tedium of collecting data and the frustration of students who obtain disappointing results for the charges on individual oil drops after two or three hours of hard work. Some novel approaches have been developed to make the data collection easier and more accurate. One method is to attach a CCD (charge coupled device) camera to the microscope of the traditional setup.^1,2 Through the CCD camera, the motion of an oil drop can be displayed on a TV monitor¹ and/or on a computer.² This allows several students to view the image of a droplet simultaneously instead of taking turns squinting through the tiny microscope eyepiece on the traditional setup. Furthermore, the motion of an oil drop can be captured and analyzed using software such as VideoPoint,³ which enhances the accuracy of the measurement of the charge on each oil drop.² While these innovative methods improve the convenience and efficiency with which data can be collected, an instructor has to invest a considerable amount of money and time so as to adapt the new techniques to his or her own classroom. In this paper, we will report on the QuickTime movies we made, which can be used to analyze the motions of 16 selected oil drops. These digital videos are available on the web⁴ for teachers to download and use with their own students. We will also share the procedure for analyzing the videos using Logger Pro,⁵ as well as our results for the charges on the oil drops and some pedagogical aspects of using the movies with students.