A graduated circle indicates the number of degrees of the deflection, which will be greater or less according to the tension of the electricity. To ascertain whether the electricity is positive or negative, a stick of shellac or glass must be employed, as already described.
Fig. 91.
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 Becquérel, 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 2½ 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 ½ 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.
“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.”
The galvanic pile employed in this electroscope is that invented by Zamboni. “It differs from the common hydro-electric batteries principally in this, that the presence of the electromotive liquid is dispensed with, and that in its place is substituted some moist substance of low conducting power, generally paper. The electromotors in these piles are composed for the most part of Dutch gold (copper) and silver (zinc) paper pressed one on the other, with their paper sides together, out of which discs are cut with a diameter of from a quarter of an inch to an inch. More powerful pairs of plates may be obtained by using only the silver paper and smearing its paper side with a thin coat of honey, on which some finely pulverized peroxide of manganese has been sprinkled, and all the sides similarly coated are presented one way. Powerful pairs of plates may also be made by pasting pure gold leaf on the paper side of zinc-paper. These plates are then to be arranged, just as in the ordinary voltaic pile, one above the other, so that the similar metallic surfaces may all lie one way; press them tightly together; tie them with pretty stout silk threads, and press them into a glass tube of convenient size. The metal rims of the tubes, which must be well connected with the outermost pairs of plates, form the poles of the pile, the negative pole being in the extreme zinc surface, and the positive in the extreme copper or manganese surface.
“The electromotive energy called into action in these dry piles is less than that excited in the moist or hydro-electric piles, principally on account of the imperfect conduction of the paper. The accumulation of electricity at their poles also goes on less rapidly, and consequently the electrical tension continues for a long while unaltered; whereas, in all moist piles, even in the most constant of them, the tension is maintained, comparatively speaking, for but a short time, on account of the chemical action and decomposition of the electromotive fluid—causes of disturbance which do not exist in the dry pile.”[15]
135. Thomson’s Electrometer.—Professor W. Thomson, of Glasgow, has devised an atmospheric electrometer, which is likely to become eminently successful, in the hands of skilful observers. It is mainly a torsion balance combined with a Leyden-jar. The index is an aluminium needle strung on a fine platinum wire, passing through its centre of gravity, and stretched firmly between two points. The needle and wire are carefully insulated from the greater part of the instrument, but are in metallic communication with two small plates fixed beside the two ends of the needle, and termed the repelling plates. A second pair of larger plates face the repelling plates, on the opposite side of the needle, but considerably farther from it. These plates are in connection with the inner coating of a Leyden-jar, and are termed the attracting plates. The whole instrument is enclosed in a metal cage, to protect the glass Leyden-jar and the delicate needle.
The Leyden-jar should be charged when the instrument is used. Its effect is two-fold: it increases greatly the sensibility of the instrument, and enables the observer to distinguish between positive and negative electrification.
The air inside the jar is kept dry by pumice-stone, slightly moistened with sulphuric acid; by which means very perfect insulation is maintained.