"Franklin's Bells" and charge transport as an undergraduate lab

"Franklin's Bells" is a popular lecture demonstration in electricity but se ems to have been overlooked as a quantitative undergraduate lab experiment. In our version, a charged ball oscillates back and forth between the plate s of a capacitor. This paper has two purposes: one is to discuss some of th e wide variety of experiments and calculations which this system affords, t he other is to present an analysis of a particular situation in which the b all excites resonant modes of the plates. This excitation gives rise to une xpected steps in the graph of shuttle frequency versus the potential differ ence between the plates. The apparatus required to show the demonstration i s available in most physics departments. Similarly, a quantitative experime nt for an introductory undergraduate lab does not require any unusual equip ment, nor particularly high voltages. (In our experiment, the highest volta ge used was 600 V; this can probably be reduced by scaling down the apparat us.) The physical situation may be analyzed at many different levels, suita ble for students in the freshman to senior years, and ranging from a qualit ative understanding of the demonstration to computer calculations of chaoti c dynamics. The apparatus may be a simple one appropriate to the introducto ry level, or, at an "Advanced Lab" level, a sophisticated one, with compute r-controlled measurements and analysis of various parameters. It is surpris ing that such a rich system has been neglected in the traditional curriculu m. (C) 2001 American Association of Physics Teachers.