The e/m experiment: Student exploration into systematic uncertainty

In this work, we convert a common verification lab in the physics curriculum, measuring the charge-to-mass ratio of the electron (e/m), into an investigative lab on systematic uncertainty. The Bainbridge apparatus, commonly used to measure e/m, can have significant systematic uncertainties, leading to large discrepancies with the accepted value. Students were asked to quantify possible systematic uncertainties in the apparatus and correct them. Building upon each other's work from semester to semester, students characterized multiple sources of systematic uncertainty. Not only did the students learn about uncertainty analysis techniques that reveal systematic uncertainties, but they also dramatically improved the accuracy of the apparatus, reducing the discrepancy from 15% to 0.5%. This paper describes a pedagogical approach to exploring unknown systematic uncertainties in an intermediate laboratory setting and the student-learning benefits of such an approach. In addition, it provides detailed information about untangling and correcting the sources of systematic uncertainty in the Bainbridge apparatus.