Electrophorus

Electrophorus, an induction machine, composed of an ebonite or resin plate, upon which a circular disc of brass or other metal may be placed by means of a glass handle. When the non-conducting plate is rubbed with fur, it becomes negatively electrified, and this negative charge remains on the surface for some time. The metal plate is then placed on it, and touched with the finger. Being in close contact with a negative charge, which cannot, however, be conducted away because of the insulation, an opposite charge is induced on the plate, the remaining negative charge on the outside being conducted away when the finger touches it. Then, when the plate is entirely removed, it will possess a residual positive charge that can be demonstrated by the small spark that escapes when the finger is again brought to it. The process may be repeated many times without apparent loss of the initial negative charge on the insulator. This is because the charge that manifests itself on the plate is not derived from the insulator at all, but practically from the earth, to which it returns when the sparking takes place. There are various modifications of the instrument, designed to illustrate the laws of induction more clearly.