Electrical charging of granular media in a shaking experiment

We present preliminary results on the electrical charging of granular media (particle size $\sim 100$ $\mu$m to $\sim 1$ mm) shaken between two conducting plates. Voltage measurements were done between the plates for both monodisperse and bidisperse sets of particles. Particle charging and electrical discharges to the plates ($\sim 1$ kV) were observed. We discuss the potential relevance to natural charging phenomena seen in sand storms, volcanic ash clouds, thunderstorms, and thundersnow. Several types of theoretical models seem plausible.