Abstract: Demonstrative apparatus of the type having a table with one or more air cushioned disks movable on the table and with apparatus for providing a record of the disk movement. The apparatus comprise a spark generator connected to the disk for generating sparks from the disk toward the table. In accordance with the present invention, the table is preferably made from insulating material. A sheet of conducting and marking material, such as colloidal carbon paper, is placed on the table beneath the disk. A sheet of plain marking paper is then placed on the colloidal carbon paper to visibly record the track of the disk as it moves over the table and as sparks are generated.