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open ring at one end, and at the other it has a brass ball, which, by a short spring socket, is slipped upon the pointed extremity, and may be removed. E is a circular piece of wood, having on its surface a slip of ivory, inlaid, and furnished with a foot, which slides into the socket F, in which it is made fast at any required height by the screw S. To this discharger belongs the small press, fig. 8, the stem of which fits into the socket, instead of the circular table E. On the top of the stem are two oblong boards, which are pressed together by means of two screws. Between these boards may be placed any substance which requires to be

ressed while the electric fluid is sent through it. The construction of this instrument is such as to enable the operator to use it with advantage in numerous experiments; particularly the oxidation of metallic leaves between slips of card paper, or of glass; splitting small pieces of oak, firing gunpowder, &c.

CoNDENSERS AND DoubleRs of Electricity.

180. Condensers of electricity are instruments used for the detection of very small portions of that matter, portions too minute to be rendered sensible by any of the electrometers which we have yet described. Several very ingenious electricians have employed themselves in the construction of instruments for this purpose: the |. of these contrivances we shall now riefly describe. 181. Volta appears to have been the first who attempted any thing of this description. His condenser of electricity consists of a flat and smooth metal plate, furnished with an insulating handle, and a semi-conducting, or imperfectly insulating, plane. When it is required to examine a weak electricity with this apparatus, as that of the air in calm and hot weather, which is not generally sensible to an electrometer, the operator must place the above-mentioned plate upon the semi-conducting plane, and a wire of some other conducting substance must be connected with the metal plate, and extended in the open air, so as to absorb its electricity; then, after a certain time, the metal plate must be separated from the plane; and, on being presented to an electrometer, it will electrify it much more than if it had not been placed upon the abovementioned plane. 182. The principle or which the action of this apparatus depends, says Mr. Cavallo, is, that the metal plate, whilst standing contiguous to the semi-conducting plane, will both absorb and retain a much greater quantity of electricity than it can either absorb or retain when separate, its capacity being increased in the former and diminished in the latter case. 182." This condenser was afterwards improved on by Mr. Cavallo, by employing a small metallic plate, about the size of a shilling, having affixed to it a glass handle covered with sealing-wax. When the larger plate appeared so slightly electrified by the communicated electricity as not to affect the electrometer, he then placed the small plate on the plane and touched it with the edge of the large one, holding the latter in an almost

vertical position; the small plate was thus found to indicate a very sensible degree of electricity. 183. One of the most convenient and elegant condensers yet contrived is that of Mr. Cuthbertson. This instrument is shown in fig. 9, and is composed of two metallic plates, a and b, about six inches in diameter, tightly screwed to two brass halls, but so as that one of them be fixed immoveably to a glass pillar, as e, while the other is fastened to a brass pillarf, having a hinge at its lower extremity by which it can be moved backwards into the position g. When the instrument is used, the electricity to be examined is communicated to the insulated plate b, while it is parallel to the uninsulated plate a, and after remaining for some time in this position the uninsulated plate is drawn back, and the intensity of the insulated plate, a, is shown by being presented to an electrometer in the usual way. 184. A modification of this instrument, called the condensing electrometer, is represented by fig. 10. In this construction the plates are smaller, and the insulated plate is attached to the cap of a gold-leaf electrometer; by this means very small degrees of electricity are dis– covered, and their intensity shown by the divergence of the gold leaves within the cylinder. 185. A condenser, of a remarkably simple description was proposed by Mr. Singer, and is constructed by placing three small spots of sealing-wax, at equal distances, on the lower face of the cover of an electrophorus, to serve as insulating feet, by which the cover may be supported at the distance of about the twelfth of an inch from the surface of a smooth and even table. If a Leyden jar, he adds, be now charged, and afterwards discharged, so as not to affect an electrometer, and its knob be then placed in contact with the condenser resting on the table for a few seconds, the small residuum of electricity in the jar will be absorbed by the condensing plate; and when this is raised from the table it will affect the electrometer with the same electricity as that with which the jar was charged. 185*. Doublers of electricity are instruments so constructed as that very small quantities of electricity may be continually doubled by them, until, being condensed, they are rendered perceptible by the common electrometer. 186. An instrument of this kind was invented by the Rev. Mr. Bennet, and is represented by fig. 1, plate V. It is formed by the addition of two polished brass plates, with insulating handles, to the common gold-leaf electrometer. The plates are varnished on the lower side, and the insulating handle of one of them, B, is fixed to the side, while that of the other, A, is placed perpendicularly in the centre. Though this apparatus appears to be very simple, it requires a very complex process, and has, therefore, generally given place to what are called moveable or revolving doublers. 187. Revolving doublers appear to have been first introduced by Dr. Darwin, who claims the merit of the invention. Darwin's doubler was moved by a train of wheels, and required to be touched by the hand to place the plates in the requisite positions. This instrument was much improved by Mr. Nicholson; a representation of

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it, in its improved form, is given in fig. 2. The whole is supported on a glass pillar six inches and a half high, and consisting of the following parts:—Two fixed plates of brass, A and C, are separately insulated and disposed in the same plane, so that a revolving plate B may pass very near them without touching. Each of these plates is two inches in diameter; they have adjusting pieces behind, which serve to place them correctly in the required position. D is a brass ball on which they turn, of two inches diameter, fixed on the extremity of an axis that carries the plate B. Besides the essential purpose which this ball is intended to answer, it is so loaded within on one side, that it serves as a counterpoise to the revolving plate, and enables the axis to remain at rest in any position. 188. The use of this ingenious instrument will be best understood from the following account of it given by Mr. Nicholson in the Philosophical Transactions:—“When the plates A and B,' says Mr. Nicholson, “are opposite to each other, the two fixed plates A and § may be considered as one mass; and the revolving plate B, together with the ball D, will then constitute another mass. All the experiments yet made concur to prove that these two masses will not possess the same electric state; but that, with respect to each other, their electricities will be plus and minus. These plates would be simple, and without any compensation, if the masses were remote from each other; but, as that is not the case, a part of the redundant electricity will assume the form of a charge in the opposed plates A and B. From other experiments I find,’ says Mr. Nicholson, “that the effect of the compensation on plates opposed to each other at the distance of one-fortieth part of an inch, is such, that they require to produce a given intensity at least 100 times the quantity of electricity that would have produced it in either, if placed singly and apart. 189. The redundant electricities in the masses under consideration, will therefore be unequally distributed : the plate A will have about ninetynine parts, and the plate C one; and, for the same reason, the revolving plate B will have ninety-nine parts of the opposite electricity, and the ball D one. The rotation, by destroying the contacts, preserves this unequal distribution, and carries B from A to C, at the same time that the tail K connects the ball with the plate C. In this situation, the electricity in B acts upon that in C, and produces the contrary state, by the communication between C and the ball; which must therefore acquire an electricity of the same kind with that of the revolving plate. But the rotation again destroys the contact, and restores B to its first situation opposite to A.’ 190. Here, if we attend to the effect of the whole revolution, we shall find that the electric states of the respective masses have been greatly Increased; for i. ninety-nine parts in A and in B remain, and the one part of electricity in C has been increased so as nearly to compensate ninety-nine parts of the opposite electricity in the revolving plate B, while the communication Produced an equal mutation in the electricity of the ball. A second rotation will of course pro

duce a proportional augmentation of these increased quantities, and a continuance of turning will soon bring the intensities to their maximum, which is limited by an explosion between the lates.’ 191. “If one of the parts be connected with an electrometer, more especially that of Bennet, these effects will be very clearly seen. The spark is usually produced by a number of turns between eleven and twenty; and the electrometer is sensibly acted upon by still fewer. When one of the parts is occasionally connected with the earth, or when the adjustment of the plates is altered, there are some variations in the effects not difficult to be reduced to the general principles, but sufficiently curious to excite the attention of persons the most experienced in this branch of natural philosophy. 192. ‘If the ball be connected with the lower art of Bennet's electrometer, and the plate A with the upper part, and any weak electricity be communicated to the electrometer, while the position of the apparatns is such that the cross piece GH touches the two pins, a very few turns will render it perceptible. But here, as in the common doubler, the effect is rendered uncertain by the condition that the communicated electricity must be strong enough to destroy and predominate over any other electricity which the plates may possess. I need scarcely observe, that, if this difficulty should be hereafter removed, the instrument will have great advantages as a multiplier of electricity in the facility of its use, the very speedy manner of its operation, and the unequivocal nature of its results.'

The ELECTRophorus.

193. Several of the instruments we have now described appear capable of collecting and imparting small quantities of electricity; but it may be questioned if any of them can be said to do so on a scale comparable with that of the common electrophorus of Volta, which may be said to be a kind of electrical machine. Fig. 3 is a representation of this simple but highly useful article of apparatus, which any person at all skilled in the use of electrical apparatus may construct for himself by attending to the following directions. Procure two circular plates of metal, or of wood covered with tin-foil, and well rounded at the edges; these are the conductors: between them is placed a resinous plate, formed by melting together equal parts of shell-lac, resin, and Venice turpentine, and pouring this mixture, whilst fluid, within a tin hoop of the required size, placed on a marble table, from which the plate may be readily separated when cold. This plate should be half an inch in thickness; it is sometimes made by pouring the mixture on one of the conductors, which is ther, formed with a rim for that purpose. In the centre of the upper conductor is fixed a glass handle of about ten inches long, for the purpose of lifting it without drawing off its electricity; and, when the electric state of the lower conductor is to be examined, the whole apparatus must be placed on an insulating stand. To use the electrophorus, rub the upper surface of the resinous plate with a piece of dry fur; cat's skin is reckoned the best, and it will be excited negatively. Place the upper conductor upon it, and then raise the same by its insulating handle; it will be found to exhibit very faint, if any, electrical signs. Replace the conductor, and, whilst it lies on the surface of the excited plate, touch it with a finger or any other uninsulated conductor, and then raise it again by its handle. It will now be positively electrified, and affordaspark: if it be then replaced on the resinous plate, touched, and again raised, another spark will be procured, and this process may be repeated for a considerable time without any perceptible diminution of effect. Jars may be charged by bringing them in contact with the conductor each time it is lifted, with an instrument of this kind only six inches in diameter. Cavallo charged a jar several times successively, and such was the strength of the charge that it was capable of piercing a card. 194. Such is the tenacity, so to speak, with which this instrument holds the electricity once excited, that some have been led to consider it as affording a perpetual source of that matter. On this opinion Mr. Cavallo makes the following just remarks: —“As to the continuance of this electric plate, when once excited without repeating the excitation, I think there is not the least foundation for believing it perpetual, as some gentlemen have supposed it to be; it being nothing more than an excited electric, it must gradually lose its power, by continually imparting some of its electricity to the air, or other substances contiguous to it. Indeed its electricity, although it could never be proved to be perpe. tual by experiments, lasts a very long time, it having been observed to be pretty strong several days, and even weeks, after excitation. The great duration of the electricity of this plate, I think, depends upon two causes: first, because it does not lose any electricity by the operation of putting the metal plate upon it, &c.; and, secondly, because of its flat figure, which exposes it to a less quantity of air, in comparison with a stick of sealing-wax, or the like, which, being cylindrical exposes its surface to a greater quantity of air, which is continually robbing the excited electrics of their virtue.' Numerous experiments are given in the writings of electricians as having been performed by the aid of this instrument; these we shall not here detail, but simply remark, that, if properly managed, it will be found capable of communicating a very high charge to tolerably sized Leyden jars.

PART III. MECHANICAL EFFECTS OF ELECTRICITY.

195. Under this part of the subject it is intended to give a popular view of a variety of electrical phenomena rather of a general and promiscuous nature, and which, from the variety necessarily involved, could not have been conveniently introduced under any other head. Among others we shall notice chiefly the following, viz. the direction of the electric fluid; its influence in expanding bodies through which it is made to pass; its power in rending solid bodies; its agency in the combustion and oxidation of me

tallic substances, and in inflaming numerous combustible substances.

DIRECTION of The Electrical FLUID.

196. From among the numerous experiments given to demonstrate, as far as it can be done the course of the electric fluid in its passages from one body to another, we select the following from Mr. Singer and others.

197. The direction of the electric fluid may be rendered visible by taking a Leyden jar that has been rendered slightly damp by being breathed on, and placing it with its knob in contact with the positive conductor of an electrical machine in a darkened room; when the jar is fully charged, if the action of the machine be continued, the fluid will be observed to pass from the internal to the external coating over the uncoated interval in luminous streams, like water overflowing from the top of a vessel kept constantly supplied. If the jar be removed, and its knob placed against the negative conductor, the stream will evidently pass in the contrary direction. A degree of dampness on the uncoated part of the glass is necessary, in this experiment, to prevent the discharge of the jar by a spontaneous explosion, in which case the fluid passes too rapidly from one surface to the other to allow the observer to ascertain its direction. If the moisture be not sufficient, diverging brushes of light will occasionally pass from the positive surface, instead of the continuous streams above mentioned.

198. Let a light wheel, the vanes of which are made of fine card-paper, be made to turn freely on its axis, a stream of electricity from a pointed wire fixed in the conductor will give it motion; and it will move from the electrified point whether its electricity be positive or negative. In this experiment the current seems to be produced by the recession of the similarly electrified air in contact with the point; and, therefore, the circumstance of the wheel turning in the same direction when the electricity is negative, cannot, as Mr. Singer has justly observed, be considered as a proof of the existence of a double current of the electric fluid.

199. Make a groove, either by bending a piece of clean card-paper, or by hollowing out a piece of baked wood, or by placing, parallel to each other, two straight sticks of sealing-wax; lay the groove upon the plate of Henley's universal discharger, and o: a large pith-ball, about half an inch in diameter, so as to be at an equal distance from the two brass knobs of the discharger. The distance of these should be about four inches, and the groove placed in the line joining the knobs. If one of the wires be now connected with the outside of a charged jar, while the knob of it is brought in contact with the other wire of the discharger, so that a small spark may pass from the one knob to the other, the pith-ball will be impelled from the positive to the negative knob. In performing the above experiment Mr. Singer used pointed wires instead of knobbed ones, and assigned, as his reason for this, that the knobs may attract the pithball, whereas the stream from the pointed wire must impel it.

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