The vapours of the atmosphere are con tinually attaching themselves to the sur face of cold glass, and by that means de stroy the electricity. Sulphur, wax, or resin, being less subject to this, retails their electric state much longer. A plate of glass or wood, coated over with any substance of this nature, may be excited by friction, and will produce electricity in a metallic plate, in the manner above described, for a very great length of time. Such a plate, together with its me tal, has been named the electropho phorus, fig. 3.
If the discharge of an electrified plate be made by the parts of a living animal, a considerable pain will be felt, chiefly at the extremities of the muscles. For ex ample, if the lower metallic plate be touched with one hand, and the other brought to the upper plate, at the instant of the emission, a pain will be felt at the wrist and elbows, which as instantly va nishes. If a larger glass plate be used, the pain will be felt on the breast ; if yet larger, the sensation will be that of a uni versal blow. This sensation has obtained the name of the shock, and will deprive animals of life, if sufficiently strong. The shock from thirty square inches of glass, well charged, will instantly kill mice, sparrows, or other small animals. Six square feet of glass will deprive a man of sensation for a time, if the head be made a part of the circuit through which the electricity moves. No inconvenience has been found from the electric shock by men of strong habits ; but women of de licate constitutions have had Convulsions from a violent shock. It may be observed, that the electric shock is a proof that the electric matter can pass through the sub stance of non-electrics, and is not uni versally conducted along the surfaces alone, as some have supposed.
The object of the philosopher being, in general, to collect a large quantity of electricity, by means of the surfaces of electrics, it is more usual to employ jars, and not plates. These are of va rious shapes and magnitudes ; but the most useful are thin cylindrical glass ves sels, about four inches in diameter, and fourteen in height, coated within and without with tin-foil, which is stuck on with gum-water, paste, or wax, excepting two inches of the rim or edge, which is left bare, to prevent the communication between the coatings. About four inches from the bottom, within, is a large cork, that receives a thick wire, ending in seve ral ramifications, which touch the inside coating; the upper end of the wire ter minating with a knob, considerably above the mouth of the jar, fig. 4. When it is
required to be charged, it may be held in the hand, or placed on an uninsulated ta• ble, and the knob of the wire applied to the conductor ; the inside coated surface becomes possessed of the electricity of the conductor, and the external surface acquires the contrary electricity, by means of its uninsulated coating. When a jar of this kind is highly charged, it will discharge spontaneously over the uncoat ed surface, and seldom through the glass; whereas, when the uncoated surface is large, it is more apt to break by that means, and become useless. Yet there is no certainty that a jar, which has dis charged itself over its surface, will not at another time break by a discharge through the glass, as the contrary often happens. If paper covered with tin-foil be used for the coating, with the paper next the glass, the jar will be less liable to break.
A jar of considerable thickness, with a neck like a bottle, in which is cemented a thick tube to receive the wire, will sus tain a very high charge, and produce much greater effects than one of the last description. The charging wire being inserted loosely into the tube, will fall out on inverting the jar, and the charge will remain for several weeks without much loss. A jar thus charged, may be put in to the pocket, and applied to many pur poses that the common jar cannot be used for.
If the inside of the jar be considerably damped, by blowing into it, through a tube reaching to the bottom, it will take 'a charge nearly one-third greater than in the ordinary state.
When a greater degree of electric force is required, larger jars must be used, in which the form is of no consequence, ex cept as fitr as relates to convenience. But it is less expensive, and nearly as effec tual, to use a number of smaller jars, hav ing the same quantity of coated surface as the large jars. In this case, a commu nication must be formed between all the outside coatings, which may be done by placing them on a stand of metal ; and also beta een all the inner coatings, which is best done by means of wires. Such a collection is called a battery, and may be charged and discharged like a single jar, fig. 5.