THE ELECTROPHORUS.
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.
If the construction of the instrument just described appears too formidable a task to my young readers, perhaps they may yet be inclined to experiment with
A SIMPLE ELECTROPHORUS.
Take a lacquered iron “tea-tray;” cut a sheet of stout brown paper so as to fit the flat part of the tray, and fix two strips of paper at each end by means of sealing-wax. These strips serve as handles by which to lift the paper, and the sealing-wax, being a non-conductor, prevents the electricity from passing off. The tray is also insulated by placing it upon two tumblers.
The sheet of paper is now heated quite hot, placed on a wooden table, and rubbed with a hard and very dry clothes-brush. Then it is lifted and placed on the tray.
The paper is negatively electrified; it induces a similar state in the lower side of the tray, which should be touched a moment with the finger; then lift the paper from the tray. An electric spark can now be taken from the latter.
The strips by which the paper is lifted can be brought together, and held by the thumb and finger of one hand, leaving the other free to take the spark. The paper may now be replaced. By touching the lower surface of the tray, and lifting the paper as before, another spark may be obtained, and so on for several times, if the air be dry.
THE EBONITE ELECTROPHORUS.
This piece of apparatus, also called Pfeiffer’s electrophorus, is composed of a thin sheet of ebonite, measuring about six by eight inches. A small sheet of brass, about five by three inches, is fixed on one side. With this, electricity may be evoked with unusual readiness.
It is placed flat on a wooden table, and rubbed successively on both sides with the open hand; if lifted in the left hand, and the right hand is presented to the brass, a spark will be received.
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