=134. Bohnenberger's Electroscope= may be fitted with a metallic conductor, and used with great advantage for observing atmospheric electricity. "The principal parts of the instrument, as improved by Becquerel, are the following:--_A B_, fig. 91, is a small dry galvanic pile of from 500 to 800 pairs, about a quarter of an inch in diameter; when the plates are pressed together, such a pile will be from 2 to 2-1/2 inches in length. The wires, which are bent so as to stand above the pile, terminate in two plates, _P_ and _M_, which are the poles of the pile. These plates, which are 2 inches by 1/2 an inch, are parallel and opposite to each other. It is convenient for their opposite sides to be slightly convex, for them to be gilded or coated with platinum, and for them to run on the polar wires, by the latter being made to pass through a small hole in them. One of these plates will always be in a state of positive, and the other of negative, electricity; between them suspend the very fine gold leaf, _D G_, which is attached to the conductor, _C D_, of copper wire. If the leaf hang exactly between the two plates, it is equally attracted by each, and will therefore be in a state of repose. The apparatus should be protected by a bell-glass, fitting exactly, and having an opening at the top through which the copper wire, _C D_, passes; the wire, however, is insulated by its being contained in a glass tube, which is made to adhere to the bell-glass by means of a small portion of shellac or gum-lac. Screw on a metal ball or plate, to impart to it the electricity you wish to test, which will be conveyed by the copper wire to the gold leaf, and the latter will immediately move towards the plate which has the opposite polarity. This electroscope is, beyond doubt, one of the most delicate ever constructed, and is well adapted to show small quantities of positive and negative electricity. [Illustration: Fig. 91.] "To ensure the susceptibility of electroscopes and electrometers placed under bell-glasses, precautions should be taken to render the air they contain as dry as possible, which may be effected by enclosing in a suitable vessel a little melted chloride of calcium beneath the glass."