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two screws.

open ring at one end, and at the other it has a vertical position; the small plate was thus found brass ball, which, by a short spring socket, is to indicate a very sensible degree of electricity. slipped upon the pointed extremity, and may be 183. One of the most convenient and elegant removed." E is a circular piece of wood, having condensers yet contrived is that of Mr. Cuthberton its surface a slip of ivory, inlaid, and fur- son. This instrument is shown in fig. 9, and is nished with a foot, which slides into the socket composed of two metallic plates, a and b, about F, in which it is made fast at any required six inches in diameter, tightly screwed to two height by the screw S. To this discharger be- brass halls, but so as that one of them be fixed longs the small press, fig. 8, the stem of which immoveably to a glass pillar, as e, while the other fits into the socket, instead of the circular table is fastened to a brass pillar f, having a hinge at its E. On the top of the stem are two oblong lower extremity by which it can be moved backboards, which are pressed together by means of wards into the position g. When the instrument

Between these boards may be is used, the electricity to be examined is complaced any substance which requires to be municated to the insulated plate b, while it is pressed while the electric fluid is sent through parallel to the uninsulated plate a, and after reit. The construction of this instrument is such maining for some time in this position the uninas to enable the operator to use it with advan- sulated plate is drawn back, and the intensity of tage in numerous experiments; particularly the the insulated plate, a, is shown by being preoxidation of metallic leaves between slips of card sented to an electrometer in the usual way. paper, or of glass; splitting small pieces of oak, 184. A modification of this instrument, called firing gunpowder, &c.

the condensing electrometer, is represented by

fig. 10. In this construction the plates are CONDENSERS AND DOUBLERS OF ELECTRICITY. smaller, and the insulated plate is attached to

the cap of a gold-leaf electrometer; by this 180. Condensers of electricity are instruments means very small degrees of electricity are disused for the detection of very small portions of covered, and their intensity shown by the dithat matter, portions too minute to be rendered vergence of the gold leaves within the cylinder. sensible by any of the electrometers which we 185. A condenser, of a remarkably simple have yet described. Several very ingenious elec- description was proposed by Mr. Singer, and tricians have employed themselves in the con- is constructed by placing three small spots of struction of instruments for this purpose: the sealing-wax, at equal distances, on the lower principal of these contrivances we shall now face of the cover of an electrophorus, to serve briefly describe.

as insulating feet, by which the cover may be 18i. Volta appears to have been the first who supported at the distance of about the twelfth of attempted any thing of this description. His an inch from the surface of a smooth and even condenser of electricity consists of a flat and table. If a Leyden jar, he adds, be now charged, smooth metal plate, furnished with an insu- and afterwards discharged, so as not to affect an lating handle, and a semi-conducting, or imper- electrometer, and its knob be then placed in confectly insulating, plane. When it is required to tact with the condenser resting on the table for a examine a weak electricity with this apparatus, few seconds, the small residuum of electricity in as that of the air in calm and hot weather, which the jar will be absorbed by the condensing plate; is not generally sensible to an electrometer, the and when this is raised from the table it will operator must place the above-mentioned plate affect the electrometer with the same electricity upon the semi-conducting plane, and a wire of as that with which the jar was charged. some other conducting substance must be con 185*. Doublers of electricity are instruments nected with the metal plate, and extended in the so constructed as that very small quantities of open air, so as to absorb its electricity; then, electricity may be continually doubled by them, after a certain time, the metal plate must be se- until, being condensed, they are rendered perparated from the plane; and, on being presented ceptible by the common electrometer. to an electrometer, it will electrify it much more 186. An instrument of this kind was invented than if it had not been placed upon the above- by the Rev. Mr. Bennet, and is represented by mentioned plane.

fig. 1, plate V. It is formed by the addition of two 182. The principle or which the action of polished brass plates, with insulating handles, to this apparatus depends, says Mr. Cavallo, is, that the common gold-leaf electrometer. The plates the metal plate, whilst standing contiguous to are varnished on the lower side, and the insuthe semi-conducting plane, will both absorb and lating handle of one of them, B, is fixed to the retain a much greater quantity of electricity than side, while that of the other, A, is placed perpenit can either absorb or retain when separate, its dicularly in the centre. Though this apparatus capacity being increased in the former and di- appears to be very simple, it requires a very minished in the latter case.

complex process, and has, therefore, generally 182.*

This condenser was afterwards improved given place to what are called moveable or reon by Mr. Cavallo, by employing a small metallic volving doublers. plate, about the size of a shilling, having affixed 187. Revolving doublers appear to have been to it a glass handle covered with sealing-wax. first introduced by Dr. Darwin, who claims the When the larger plate appeared so. slightly elec- merit of the invention. Darwin's doubler was trified by the communicated electricity as not to moved by a train of wheels, and required to be affect the electrometer, he then placed the small touched by the hand to place the plates in the plate on the plane and touched it with the edge req positions. This instrument was much of the large one, holding the latter in an almost improved by Mr. Nicholson; a representation of

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it , in its improved form, is given in fig. 2. The duce a proportional augmentation of these inwhole is supported on a glass pillar six inches creased quantities, and a continuance of turning and a half high, and consisting of the following will soon bring the intensities to their maximum, parts :—Two fixed plates of brass, A and C, are which is limited by an explosion between the separately insulated and disposed in the same plates.' plane, so that a revolving plate B may pass very 191. . If one of the parts be connected with near them without touching. Each of these an electrometer, more especially that of Bennet, plates is two inches in diameter; they have ad- these effects will be very clearly seen. The justing pieces behind, which serve to place them spark is usually produced by a number of turns correctly in the required position. D is a brass between eleven and twenty; and the electrometer ball on which they turn, of two inches diameter, is sensibly acted upon by still fewer. When one fixed on the extremity of an axis that carries the of the parts is occasionally connected with the plate B. Besides the essential purpose which earth, or when the adjustment of the plates is this ball is intended to answer, it is so loaded altered, there are some variations in the effects within on one side, that it serves as a counter- not difficult to be reduced to the general principoise to the revolving plate, and enables the axis ples, but sufficiently curious to excite the attento remain at rest in any position.

tion of persons the most experienced in this 188. The use of this ingenious instrument will branch of natural philosophy. be best understood from the following account 192. If the ball be connected with the lower of it given by Mr. Nicholson in the Philosophical part of Bennet's electrometer, and the plate A Transactions :

—When the plates A and B,' says with the upper part, and any weak electricity be Mr. Nicholson, are opposite to each other, the communicated to the electrometer, while the two fixed plates A and C may be considered as position of the apparatns is such that the cross one mass; and the revolving plate B, together piece G H touches the two pins, a very few turns with the ball D, will then constitute another will render it perceptible. But here, as in the mass. All the experiments yet made concur to common doubler, the effect is rendered uncertain prove that these two masses will not possess the by the condition that the communicated electrisane electric state; but that, with respect to city must be strong enough to destroy and preeach other, their electricities will be plus and dominate over any other electricity which the minus. These plates would be simple, and plates may possess.

I need scarcely observe, without any compensation, if the masses were that, if this difficulty should be hereafter removed, remote from each other; but, as that is not the the instrument will have great advantages as a case, a part of the redundant electricity will as- multiplier of electricity in the facility of its use, sume the form of a charge in the opposed plates the very speedy manner of its operation, and the A and B. From other experiments I find,' says unequivocal nature of its results.' Mr. Nicholson, 'that the effect of the compen

The ELECTROPHORUS. sation on plates opposed to each other at the distance of one-fortieth part of an inch, is such, that

193. Several of the instruments we have now they require to produce a given intensity at least described appear capable of collecting and im100 times the quantity of electricity that would parting small quantities of electricity; but it may have produced it in either, if placed singly and be questioned if any of them can be said to do apart.

so on a scale comparable with that of the com189. The redundant electricities in the masses mon electrophorus of Volta, which may be said under consideration, will therefore be unequally to be a kind of electrical machine. Fig. 3 is a distributed : the plate A will have about ninety- representation of this simple but highly useful nine parts, and the plate Cone; and, for the article of apparatus, which any person at all same reason, the revolving plate B will have skilled in the use of electrical apparatus may ninety-nine parts of the opposite electricity, and construct for himself by attending to the follow

The rotation, by destroying the ing directions. Procure two circular plates of contacts, preserves this unequal distribution, and metal, or of wood covered with tin-foil, and well carries B from A to C, at the same time that the rounded at the edges; these are the conductors : tail K connects the ball with the plate C. In between them is placed a resinous plate, formed this situation, the electricity in B acts upon that by melting together equal parts of shell-lac, in C, and produces the contrary state, by the resin, and Venice turpentine, and pouring this communication between C and the ball ; which mixture, whilst fluid, within a tin hoop of the must therefore acquire an electricity of the same required size, placed on a marble table, from kind with that of the revolving plate. But the which the plate may be readily separated when rotation again destroys the contact

, and restores cold. This plate should be half an inch in B to its first situation opposite to A.'

thickness; it is sometimes made by pouring the 190. Here, if we attend to the effect of the mixture on one of the conductors, which is ther. whole revolution, we shall find that the electric formed with a rim for that purpose. In the censtates of the respective masses have been greatly tre of the upper conductor is fixed a glass handle increased; for the ninety-nine parts in A and in of about ten inches long, for the purpose of liftBremain, and the one part of electricity in C ing it without drawing off its electricity; and, has been increased so as nearly to compensate when the electric state of the lower conductor is ninety-nine parts of the opposite electricity in to be examined, the whole apparatus must be the revolving plate B, while the communication placed on an insulating stand." To use the elecproduced an equal mutation in the electricity of trophorus, rub the upper surface of the resinous the ball. A second rotation will of course pro- plate with a piece of dry fur; cat's skin is

the ball D one.

reckoned the best, and it will be excited nega- tallic substances, and in inflaming numerous tively. Place the upper conductor upon it, and combustible substances. then raise the same by its insulating handle; it will be found to exhibit very faint, if any, elec

DIRECTION OF THE ELECTRICAL Fluid. trical signs. Replace the conductor, and, whilst 196. From among the numerous experiments it lies on the surface of the excited plate, touch it given to demonstrate, as far as it can be done with a finger or any other uninsulated conductor, the course of the electric fluid in its passages from and then raise it again by its handle. It will one body to another, we select the following from now be positively electrified, and afford a spark: if Mr. Singer and others. it be then replaced on the resinous plate, touched,

197. The direction of the electric fluid may be and again raised, another spark will be procured, rendered visible by taking a Leyden jar that has and this process may be repeated for a consider- been rendered slightly damp by being breathed able time without any perceptible diminution on, and placing it with its knob in contact with of effect. Jars may be charged by bringing the positive conductor of an electrical machine them in contact with the conductor each time it in a darkened room; when the jar is fully is lifted, with an instrument of this kind only six charged, if the action of the machine be coninches in diameter. Cavallo charged a jar seve- tinued, the fluid will be observed to pass from ral times successively, and such was the strength the internal to the external coating over the unof the charge that it was capable of piercing a

coated interval in luminous streams, like water card.

overflowing from the top of a vessel kept con194. Such is the tenacity, so to speak, with stantly supplied. If the jar be removed, and its which this instrument holds the electricity once knob placed against the negative conductor, the excited, that some have been led to consider it stream will evidently pass in the contrary direcas affording a perpetual source of that matter. tion. A degree of dampness on the uncoated On this opinion Mr. Cavallo makes the following part of the glass is necessary, in this experiment, just remarks: -- As to the continuance of this to prevent the discharge of the jar by a spontaelectric plate, when once excited without repeat- neous explosion, in which case the fluid passes ing the excitation, I think there is not the least too rapidly from one surface to the other to allow foundation for believing it perpetual, as some the observer to ascertain its direction. If the gentlemen have supposed it to be; it being moisture be not sufficient, diverging brushes of nothing more than an excited electric, it must light will occasionally pass from the positive surgradually lose its power, by continually impart- face, instead of the continuous streams above ing some of its electricity to the air, or other sub- mentioned. stances contiguous to it. Indeed its electricity,

198. Let a light wheel, the vanes of which are although it could never be proved to be perpe. made of fine card-paper, be made to turn freely tual by experiments, lasts a very long time, it on its axis, a stream of electricity from a pointed having been observed to be pretty strong several wire fixed in the conductor will give it motion; days, and even weeks, after excitation. The and it will move from the electrified point whegreat duration of the electricity of this plate, I ther its electricity be positive or negative. In this think, depends upon two causes : first, because experiment the current seems to be produced by it does not lose any electricity by the operation the recession of the similarly electrified air in of putting the metal plate upon it, &c.; and, contact with the point; and, therefore, the cirsecondly, because of its flat figure, which ex cumstance of the wheel turning in the same poses it to a less quantity of air, in comparison direction when the electricity is negative, cannot, with a stick of sealing-wax, or the like, which, as Mr. Singer has justly observed, be considered being cylindrical exposes its surface to a greater as a proof of the existence of a double current of quantity of air, which is continually robbing the the electric fluid. excited electrics of their virtue.' Numerous 199. Make a groove, either by bending a experiments are given in the writings of electri- piece of clean card-paper, or by hollowing out a cians as having been performed by the aid of piece of baked wood, or by placing, parallel to this instrument; these we shall not here detail, each other, two straight sticks of sealing-wax; but simply remark, that, if properly managed, it lay the groove upon the plate of Henley's uniwill be found capable of communicating a very about half an inch in diameter, so as to be at an

versal discharger, and place a large pith-ball, high charge to tolerably sized Leyden jars.

equal distance from the two brass knobs of the PART III.

discharger. The distance of these should be

about four inches, and the groove placed in the MECHANICAL EFFECTS OF ELECTRICITY.

line joining the knobs. If one of the wires be 195. Under this part of the subject it is in- now connected with the outside of a charged jar, tended to give a popular view of a variety of while the knob of it is brought in contact with electrical phenomena rather of a general and the other wire of the discharger, so that a small promiscuous nature, and which, from the variety spark may pass früm the one knob to the other, necessarily involved, could not have been conve- the pith-ball will be impelled from the positive niently introduced under any other head. Among to the negative knob. In performing the above others we shall notice chiefly the following, viz. experiment Mr. Singer used pointed wires the direction of the electric Auid ; its influence instead of knobbed ones, and assigned, as his reain expanding bodies through which it is made son for this, that the knobs may attract the pithto pass ; its power in rending solid bodies; its ball, whereas the stream from the pointed wire agency in the combustion and oxidation of me- must impel it.

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