“Franklin’s Bells” and charge transport as an undergraduate lab

Franklin's Bells is a popular lecture demonstration in electricity but seems to have been overlooked as a quantitative undergraduate lab experiment. In our version, a charged ball oscillates back and forth between the plates of a capacitor. This paper has two purposes: one is to discuss some of the wide variety of experiments and calculations which this system affords, the other is to present an analysis of a particular situation in which the ball excites resonant modes of the plates. This excitation gives rise to unexpected steps in the graph of shuttle frequency versus the potential difference between the plates. The apparatus required to show the demonstration is available in most physics departments. Similarly, a quantitative experiment for an introductory undergraduate lab does not require any unusual equipment, nor particularly high voltages. (In our experiment, the highest voltage used was 600 V; this can probably be reduced by scaling down the apparatus.) The physical situation may be analyzed at many different levels, suitable for students in the freshman to senior years, and ranging from a qualitative understanding of the demonstration to computer calculations of chaotic dynamics. The apparatus may be a simple one appropriate to the introductory level, or, at an "Advanced Lab" level, a sophisticated one, with computer-controlled measurements and analysis of various parameters. It is surprising that such a rich system has been neglected in the traditional curriculum. (C) 2001 American Association of Physics Teachers.