If ordinary unglazed paper be immersed in a mixture of equal parts of sulphuric and nitric acids, then well washed with plenty of water and dried, it becomes extremely electric. If placed on a wooden table, or, better still, on a waxed cloth, and rubbed with the hand, it attracts feathers, pith-balls, fragments of paper, or other small light objects.
When suddenly stripped from the waxed cloth in a darkened room, the entire surface will have a luminous phosphorescent appearance. A spark can be taken from it by holding the finger about half an inch from the surface. If placed against the wall it will adhere to it and keep its place for several minutes.
This paper retains its electrical properties a long time. When weakened, it is sufficient to slightly heat it to restore all its energy.
This instrument, whose name, derived from the Greek, means _bearer of electricity_, consists of two parts; first, a cake or disk of resin, or of shellac and wax, these substances being melted and poured into a tin mold; second, a disk of brass, or sometimes of thin, well-dried wood, covered on each side with thin sheet-brass or even thick tin-foil. This should be fitted with a glass handle, to insulate it; a stout, round bottle of moderate size will answer. The cake of resin is rubbed vigorously; a surface of fur is the best to use for this, such as a cat-skin or fox-tail. The disk is then taken by the handle and rested on the cake, and its upper surface touched a moment with the finger; then, on withdrawing the disk from the resin, a bright electric spark can be obtained from it. By resting it once more on the resin, again touching and withdrawing it, another spark may be elicited, and so on for eight or ten successive trials.
The scientific explanation of this phenomenon is, that negative electricity is excited in the cake by friction. When the disk is applied, the electricity does not pass into it from the cake, but is _induced_ in the disk by the law of electrical polarity; the lower surface being covered with positive electricity, while the negative is repelled to the upper side, from which it is drawn by the finger. Then, when the disk is lifted, the spark of positive electricity may be drawn.