The Effects of a Magnetic Field On a Conical Bead Pile

Lilianna Christman


The effects of increasing cohesion between steel beads in a slowly-driven bead pile were investigated by surrounding the pile with two Helmholtz coils to produce a uniform magnetic field. When a magnetic field is present, the beads become magnetized but do not stay magnetized when the coil current is reduced, enabling us to change the strength of the magnetic field and thus the cohesion between the beads as desired. Data were taken at four different field strengths (currents of 0 mA, 500 mA, 750 mA) with drop heights varying from 2 cm up to 8 cm. The data were analyzed to examine the probability of avalanches based on size. The effects of cohesion matched preliminary results found previously at the College of Wooster. When there was cohesion among the beads, the probability of large avalanches increased while the probability of mid-sized avalanches decreased, causing humps in the probability distribution. As the cohesion increased, the humps became larger and occurred at higher avalanche sizes. In addition, the largest avalanches seen with high cohesion were larger than the largest avalanches seen with low or no cohesion. The data were also analyzed to examine the probability distribution of inter-event times in bead drops for different size avalanches. For large avalanches, the inter-event times were longer. The typical inter-event time also became longer when cohesion was increased. The high drop height somewhat counteracted this effect by shortening the typical inter-event time.