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quently those to which this modus operandi must be traced, and to which alone it can be attributable. The theoretical views of electricity which I have taken, are clearly described in my first memoir (40–87),* and are those alone which it will be necessary to apply on the present occasion, in forming the basis of the theory of voltaic electricity.
372. In the explanation of the principles of this theory I shall commence with the simplest case, hoping that the reader will bear in mind all that I have said, in my first memoir, respecting the unequable distribution of the electric fluid amongst the bodies composing the surface of the earth, and as far within its body as has hitherto been explored. But, in order to avoid controversy, it may be well to limit the range of my views to those bodies constituting the surface only. 373. Since, then, equal volumes of all
Fig. 1. the different kinds of matter, whilst surrounded by an equable electric pressure, are charged with different quantities of the electric matter, proportional to their susceptibilities of receiving it, it is obvious that those quantities would vary with every variation of the electric pressure, whether that variation of pressure were general on every side, or partial only, by its limitation to one particular portion of the surface. Let us, for instance, apply this reasoning to the well known experiment with Volta’s plates of copper and zinc, than which, I know of no simpler case. See Fig. 1.
374. Prior to bringing the plates into contact with each other, each plate is surrounded by an equable electric pressure; and, consequently, each has its natural share of the electric fluid, due to its susceptibility of receiving it, under such pressure. But when the plates are brought into contact with each other, face to face, it is very obvious that the electric pressure, in the plane of contact, bas received a material alteration : it is, in fact, lessened on the face of the copper, but increased on the face of the zinc: and a new distribution of the electric fluid takes place, iu consequence of a momentary flow from the copper, through the plane of contact, 10 the zinc. The zinc having now received more of the fluid than it had whilst under the natural equable pressure, and the copper having lost that quantity, the former becomes positively, and the latter negatively electrical. The distribution, however, is now of a peculiar kind, and very different to the distribution on each individual body prior to contact: which, was equable on every side. The new distribution brings the pair of metals into an electro-polar state; the exterior surfaces of the zinc and copper being, respectively, positively and negatively electrical; not only with respect to each other, but with respect to the plane of contact, and also to exterior bodies
• Annals of Electricity, rol. ii, page 401.
whose natural circumambient electric pressure has not been disturbed
375. If the metals be suddenly separated, whilst insulated, the redundant fluid on the zinc has not time to return to the copper: the for. mer is, therefore, left in a positively, and the latter in a negatively electrical condition ; but, if the separation be made slowly, the whole, or nearly the whole, of the redundant fluid will return to the copper plate: and the usual equilibriums on the two metals will be restored.
376. Now, since it is an invariable law in electricity, that no flow of the electric fluid can take place from one body to another, unless the former be positive to the latter,* the experiment with Volta's discs furnishes us with a piece of interesting information, which, independently of some such experiment, we had no means of arriving at. It shows us in the most decisive manner, that prior to contact, and whilst the metals were subjected to the same electric
pressure, the copper was positive to the zinc; but that, after contact, the zinc was positive to the copper.t By proceeding cautiously, with similar experiments, on different bodies, the relative natural electric states of an extensive series of them might easily be obtained : this interesting piece of information, however, remains a desideratum, with the exception of a very few insulated facts.
377. Ii, whilst the copper and zinc plates were in contact, a copper wire were to connect their outer surfaces, a return of a small portion of the electric fluid, from the zinc to the copper, would take place, and a new distribution, and subsequent equilibrium, would be the results. But, as the transverse sectional area of the conducting arc, which rested on the surface of the zinc, would be extremely small, when compared to the whole area of that surface, the new distribution of the fluid, produced by the application of the arc, would be very little different to that displayed prior to such application : and would not be productive of any change in the general character of the polarization.
378. if, however, the conducting arc were of large transverse dimensions, or if a great number of small ones were employed, so as to cover, by their sectional areas, a large portion of the surface of the zinc plate, the previous distribution, due to the contact of the copper plate, would be very inuch altered. Nevertheless, the previously displayed polarization, although much lessened in degree, would still retain its original character on every portion of the exterior surface of the zinc that remained unoccupied by the ends of the conducting arcs; and it is not until the whole surface of the zinc is completely covered with copper, that the polarization ceases to be displayed.
I believe that this fact had never been noticed by any writer on electricity antil I mentioned it in my Familiar Instructions in the Theory and Practice of Electro-Gilding, Sc., published in March last.
t I am well aware that many persons are of opinion that zinc is invariably positive to copper, but there are rery few of them, if any, who have giren a reason for entertaining that opinion.
379. I have made extensive series of experiments on this delicate part of the subject, and from their results I have been led to conclude, that these facts are not limited to the employment of copper and zinc only, but that they are producible, and can be satisfactorily displayed by any two metals whatever: and not only by the employment of two distinct kinds of metal, but by two discs of one individual metal, cast from one and the same fused mass.
380. Now, when the polarization first takes place, by the simple contact of Volta's plates, and the new equilibrium has become established, it is obvious that the electric force on the zinc surface has become increased, and that on the copper surface diminished. Hence it is, that the respective electric tendencies of those surfaces, with reference to the circumainbient medium, are very different: the zinc having a tendency to dispose of a portion of its fluid, and the copper a corresponding tendency to receive a portion. Under these circumstances we have an opportunity of either increasing or diminishing the sum total, or general stock, of the electric fluid in the two plates. If, for instance, I touch the copper plate with an uninsulated conductor, a new portion of Auid is transmitted to its previously negative surface, and the general stock is increased: but if, on the contrary, I touch the exterior surface of the zinc plate with a similarly situated conductor, a portion of the fluid, previously accumulated on that surface, is delivered over to the conductor, and the general stock becomes diminished. A moment's contact with the conductor, in each case, is sufficient to produce the effects stated: which may be proved by separating the plates slowly afterwards. In the former case both plates are found to be positively electrical; and in the latter case, both are found to be negatively electrical.*
381. The principles on which the display of the above described phenomena depend, may be conveniently taken advantage of in exhibiting the electric conditions of the plates by their simple contact: because the action of each individual plate is enhanced by applying a conductor to the other, whilst they are in contact. If, for instance, I wish to show the positively electric action of the zinc to the greatest advantage, I insulate that plate only, and not the copper: and, on the other hand, if I wish to show the negative electric action of the copper plate to advantage, I insulate it only, and keep my hand in contact with the zinc. By these means, a single contact is sufficient to show the electric character of each plate.
382. Hitherto I have noticed the electro-polarization of the zinc and copper plates, only as a group, or a collective whole; but it will be easily understood that, since the exterior surfaces of the zinc and copper are respectively positive and negative to the plane of contact, each individual plate is also electro-polar: the outer and inner sur
For the performance of these experiments the most delicate electroscope must be employed. That described at page 427 of this volume, is the one used in my investigations.
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 Auid, 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 Auid on that surface of B will be increased. The disturbance of the Auid 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.
391, When one extremity of the pile is held in the hand, and 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 instruments indicate electric action in the extremities of the column, the one positive and the other negative. By a series of 20,000