"Plum pudding": Corks with magnets float in bowl/solenoid on OHP.

"Plum pudding": Corks with magnets float in bowl/solenoid on OHP. J.J. Thompson envisioned an atom to be made up of a small sphere of uniform positive charge within which negatively charged electrons were distributed, rather like plums in plum pudding. The positive charge would drive the electrons inward, and the electric charges would repel each other, arranging themselves in 'shells'. A simple model to demonstrate this concept two-dimensionally is to float corks with magnetized needles in a dish of water immersed in a magnetic field. This demo uses a clear glass dish about 25 cm in diameter wound with about 30 turns of insulated copper wire. A six volt battery is attached to the coil through a variable resistor. The glass dish is filled with enough water to float the corks, with a few milliliters of methyl alcohol added to cut surface tension. (Care must be taken that all the cork needles are magnetized in the same direction.) The magnetic field is switched on so that each cork is driven toward the center of the dish. As corks are added, a stable pattern of rings becomes evident. A decrease or increase in current will cause the patterns to shrink or increase, corresponding to a lesser or greater positive charge. The apparatus is on an overhead projector enabling class to view the shadow projections. Ref.:Modern College Physics by Harvey White, 6th ed., p. 663