Electrophorous

The electrophorous was invented by Alessandro Volta, it comprises a disk of insulating material resting on a metal base, and another metal disk mounted on the end of an insulated handle. Traditionally the insulating disk was made from a hard wax or resin, which was beaten with a piece of fur, frequently cat fur was used. The insulating disk was beaten or rubbed vigorously with the fur, causing an electrostatic charge to be formed.

Fig 1	Fig 2	Fig 3	Fig 4

The charge produced on the insulating disc is negative (fig1). The top disk is then placed in contact with it, because the surfaces are only able to touch at a relatively few points, a positive charge is induced on the lower surface, and a corresponding positive charge is produced on its top surface (fig 2). The top of the upper disk is then touched briefly with a finger, carrying away the negative charge (fig3). This leaves a net positive charge on the top disk (fig4). The top disk may be repeatedly charged in the same way, in theory one could do it forever, but the insulated disk does slowly discharge to the surroundings.