Mr. K.L. Bauer, of Carlsruhe, has just constructed a very simple and ingenious apparatus which permits of demonstrating that electricity develops only on the surface of conductors. It consists (see figure) essentially of a yellow-metal disk, M, fixed to an insulating support, F, and carrying a concentric disk of ebonite, H. This latter receives a hollow and closed hemisphere, J, of yellow metal, whose base has a smaller diameter than that of the disk, H, and is perfectly insulated by the latter. Another yellow-metal hemisphere, S, open below, is connected with an insulating handle, G. The basal diameter of this second hemisphere is such that when the latter is placed over J its edge rests upon the lower disk, M. These various pieces being supposed placed as shown in the figure, the shell, S, forms with the disk, M, a hollow, closed hemisphere that imprisons the hemisphere, J, which is likewise hollow and closed, and perfectly insulated from the former.
The shell, S, is provided internally with a curved yellow-metal spring, whose point of attachment is at B, and whose free extremity is connected with an ebonite button, K, which projects from the shell, S. By pressing this button, a contact may be established between the external hemisphere (formed of the pieces, S and M), and the internal one, J. As soon as the button is left to itself, the spring again begins to bear against the interior surface of S, and the two hemispheres are again insulated.
The experiment is performed in this wise: The shell, S, is removed. Then a disk of steatite affixed to an insulating handle is rubbed for a few instants with a fox's "brush," and held near J until a spark occurs. Then the apparatus is grasped by the support, F, and an elder-pith ball suspended by a flaxen thread from a good conducting support is brought near J. The ball will be quickly repelled, and care must be taken that it does not come into contact with J. After this the apparatus is placed upon a table, the shell, S, is taken by its handle, G, and placed in the position shown in the figure, and a momentary contact is established between the two hemispheres by pressing the button, K. Then the shell, S, is lifted, and the disk, M, is touched at the same time with the other hand. If, now, the pith ball be brought near S, it will be quickly repelled, while it will remain stationary if it be brought near J, thus proving that all the electricity passed from J to S at the moment of contact.--La Lumiere Electrique.