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.

[Illustration]

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