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faces of the zinc, and the inner and outer surfaces of the copper, being positive and negative respectively.

383. Now, although the two surfaces of a zinc plate would not become, relatively, in positive and negative states from the contact of each with a plate of copper of similar dimensions, their electrical conditions would become changed, and the whole group would become electro-polar: both collectively and individually, as decidedly as in any other case. To show this fact satisfactorily, the zinc plate ougnt to be pretty thick, and insulated: and the copper plates held in the hands by metallic handles. When the contact is completed on both sides of the zinc, and the copper plates afterwards suddenly and simultaneously separated from it, it is invariably left in a positively electric state. The fact is also shown by reversing the experiment; and operating with insulated copper plates and uninsulated zinc. Let the zinc plate be supported on an uninsulating pillar, with its plane vertical: and let each of its surfaces be covered with an insulated copper plate. On separating the latter plates suddenly from the zinc they will be found negatively electrical.

384. Nothing could be more satisfactory in establishing a general law in electricity than the facts here stated: viz. that, by the simple contact of dissimilar metallic bodies, a partial transfer of the electric fluid from one to the other invariably takes place. This is not only a general law in electricity, but also one of the fundamental laws in Voltaism, or voltaic electricity, in all those associations in which two dissimilar metals are employed: and the same law is applicable in all other voltaic associations, whatever may be the character of the materials which enter into them.

385. Therefore the first step in every case of voltaic electric action, is a transfer of electric fluid from one of the bodies to another. This first move of the electric fluid gives rise to a new electrodistribution to electro-polarization, and, in those cases where the group is insulated, to a new electro-equilibrium.

386. Having thus satisfied ourselves respecting the primary and the secondary electric conditions of each individual pair of metals, our next consideration is to ascertain what takes places in a series of pairs placed within the'sphere of each others action, the simplest of which is that of the dry pile.

387. When two pairs, A and B, are placed in such a manner, with respect to each other, that the positive surface of A be directly opposite the negative surface of B, having only a thin film of air between them, the previous electro-equilibrium of each pair will again be disturbed; for the accumulated fluid, on the inner positive surface of A, will charge the thin film of air, and thus cause it to exert a greater electric pressure on the vicinal negative surface of B than that to which it was previously exposed; and no corresponding pressure taking place on the exterior or positive surface of this latter pair, its fluid will be urged in that direction, and, consequently, the accumulation of fluid on that surface of B will be increased. The disturbance of the fluid in A arises from the electric

pressure on its inner or positive surface being diminished by the vicinal negative surface of B, without any corresponding diminution of pressure on its outer surface, the first effect of which is a movement of the fluid in A towards B. It therefore appears that the fluid, in both pairs, moves in one and the same direction, and that the ultimity is a new equilibrium in the group, in which a more powerful electro-polarity is established than can be displayed by either pair alone.

388. It will now appear very obvious, that, if a third pair C, were to be added to A and B, the electro-polarity at the extremities of the series would become still greater than that displayed by a group of two pairs only; and, for the same reason, every additional pair would cause an increase of polarity in the series which, when extended to about 100 pairs, would be sufficiently powerful to affect electroscopes, and put light pendulous bodies into motion.

389. The electro-polarization of bodies may be enhanced either by augmenting the disturbing force, or by lessening the resistance of the surrounding mediums. The latter circumstance is usually resorted to in the construction of the electric column, in which discs of dry paper, instead of films of air, form the intermediate medium between the metallic pairs.

390. M. Marechaux was the first philosopher who employed paper in the dry electric column. The metals in M. de Luc's columns were discs of thin zinc, and of Dutch gilt paper; the gilt side of the paper being in contact with the zinc in every pair. The series was strung upon a silken thread, which passed through the centre of the whole, and then placed in a glass tube furnished with brass caps and hooks at its extremities, as represented by Fig. 2.

Fig. 2.

391. When one extremity of the pile is held in the hand, aud the other to the cap of an electroscope, the gold leaves immediately diverge, indicating the electric character of the pole in contact with the instrument. Or the column may be placed horizontally on the caps of two gold leaf electroscopes, as represented by Fig. 3. In this case both in

Fig. 3.

struments indicate electric action in the extremities of the column, the one positive and the other negative. By a series of 20,000

pairs the late Mr. Singer was enabled to charge Leyden jars by a moment's contact. By the employment of coated talc, instead of glass, I find that an extensive surface may be charged by a dry pile of 10,000 pairs.*

392. The principal consideration, in this place, is the perpetuity of the action of the dry electric column; which, when properly constructed, and securely guarded against moisture, appears to be possessed of interminable electric powers. At the time when philosophers, especially the chemical part of them, could form no idea of the changes in the electric characters of bodies but such as emanated from oxidation, the most fruitful of all sources of error, in this branch of physics, the dry electric column shared the same fate as the wet pile of Volta, in having its action placed to the credit of oxidation. And even when it was found that the dry column would retain its action for several years, a slow oxidation of the metals was the only explanation that could be given for the display of its electricity. This idea became so fashionable eventually, that it became the basis of the prevailing hypothesis which, to this day, is strongly contended for by certain philosophers now in controversy on this interesting topic.

393. To those who have paid attention to the succession of steps that I have hitherto taken in tracing the electric action from metal to metal in each individual pair, and also that action which is due to a series of pairs, as associated in the dry electric column, there can appear no reason whatever for calling into the hypothesis an oxidation of the metals; but, on the contrary, since purity of the metallic surfaces is essential to the development of those peculiar arrangements of the electric forces which I have described, and which form the very soul of the dry electric column; and since the introduction of an oxidizing process would soon pollute these metallic surfaces, and ultimately destroy every particle of their metallic character, nothing can appear more evident in physical science, than that a perpetuity of electric action in the dry pile, depends upon a continuance of the purity of the metal.

394. The inference thus drawn from the simple principles of electricity, is that alone which can direct the practical electrician with certainty to the construction of a permanently acting electric column. It is conformable with all experience, and, I believe, is now acted upon by those who make the best apparatus of this kind. Every care is taken to insulate the elements of the column from the atmospheric air, and from every kind of moisture; which, when properly accomplished, the permanency of electric action becomes secured.

395. As the whole action of the column depends upon electric pressures, any change in the external electric pressure of the atmos

The whole process of forming dry electric columns, with a variety of their interesting applications, will be clearly described in my Cyclopædia of Electricity and Magnetism, shortly to be published.

The

Fig. 4.

phere will necessarily affect the action of the apparatus. It is on this account that it becomes an indicator of those fluctions of atmospheric electrical pressure which are almost continually going on; and, as these fluctuations are occasioned by change of temperature, humidity, evaporation, winds, barometric pressures, clouds, &c., and frequently by several of these causes at the same time, there can be no astonishment excited at M. de Luc's failures in attempting to apply the electric column as a meteorological instrument. frequent changes in the activity of this apparatus, first noticed by that ingenious and indefatigable philosopher, are exceedingly curious and interesting. He attached to the upper pole of a vertical electric column, a bent wire, which reached downwards as low as the lower pole, and terminated in a small brass ball. Between this ball and the lower pole of the column was suspended, by a silken fibre, a light gilt pith ball; the whole being covered by a glass shade, as represented in Fig. 4. A pendulous ball vibrates between the lower pole of the column and the opposite ball, carrying the electric fluid from one to the other continually. When the atmosphere is highly charged with the electric fluid the ball vibrates with great rapidity; it also moves rapidly in a warm room, or any warm unattenuated atmosphere, but languishes very materially in a moist atmosphere. 396. When two columns are arranged as in Fig. 5, having a bell attached to each lower pole, the one positive, and the other negative, the pendulous brass ball plays between them, and rings both bells; thus warning the observer when any material atmospheric electrical change is taking place. Apparatus of this kind have kept in play for upwards of twenty years, and are still as active as at first; and there can be no satisfactory reason shown why they should ever cease to display their electric forces.

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Fig. 5.

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397. Having now disposed of the views which I have taken of the theory of the dry electric column, I shall next endeavour to explain the principles upon which the wet pile, or voltaic battery, operates; and why its action soon languishes, and ultimately becomes annihilated. And in this attempt, as in that of the previous part of the subject, I shall proceed with the simplest case.

398. The simplest case in the action of the wet pile of Volta, is similar to that of the dry pile, already discussed; and the action of such a pile, under certain circumstances, would be as durable as that of the pile of M. de Luc, or of any dry pile whatever, provided the

electric condition of each individual metallic surface were uniform; but, in consequence of an ununiformity of electric action on various points of every metallic surface, as already explained in the first section of this memoir, any moisture, in contact with such surface, would suffer partial decomposition, and the metal itself undergo a change, whether employed as an element in a voltaic series or not.

399. When, however, pure water is employed for the intervening medium between the metallic pairs, the pile assumes electro-polarity in a supereminent degree: and a series of about five hundred pairs would charge an extensive coated surface of glass, by one momentary contact of either of its poles. Five hundred square feet of coated glass have been charged, by a pile of this description, to a higher degree of intensity than can possibly be accomplished, in the same time, by the most powerful electric machine: and by increasing the extent of the voltaic series, the charge would Fig. 6. increase in proportion. Hence, from a source of this kind, electricity, to almost any required extent, might be obtained. And, although the local electric action, especially on the surface of the zinc, tends continually to lessen the general electric forces of the pile, and eventually annihilates them altogether, the apparatus retains a considerable degree of action for many successive hours. The theory of the general action of this apparatus is precisely that already given for the dry electric column; and it is a matter of no consequence, whether the metals be piled one upon another, with intervening moistened paper, as represented by Fig. 6, or that they be placed edgewise, in a Cruikshank's trough, represented by Fig. 7, whose cells are

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newly filled with pure water. In this latter form, however, the cells must be perfectly water-tight, and the pairs of metal well insulated from each other by resinous cement.

400. When the voltaic pile, or battery, is employed in its most usual capacity, for the production of electric currents, the principles hitherto described form a part only of the theory of its action; for stable electro-equilibrium is inconsistent with the idea of a continuous flowing current; therefore, other principles must enter the framework of the theory, in order that its explanations may become satisJactorily applicable to every variety of case.

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