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above, cannot relate to the expansive power of the electric fluid; for, between two bodies in mutual contact, such a power must be in equilibrium; therefore these expressions must relate to density, such as I have defined it. Let us now attend to the experiments.
These trials are usually made with discs of zinc and copper (or brass) having an insulating handle in their centre, like the plate of an electrophorus, and with the help of the condenser ; thus, one of the discs being held on the hand, the other is first laid upon it, then brought, by its insulating handle, into contact with the condenser. It is commonly supposed that the latter must be touched while it lies on the other; but this, assimilating the process to that of the electrophorus, changes the nature of the phenomenon, and may lead into error. I shall therefore first relate the experiments which I have made, with discs of the two metals about four inches diameter, from which the difference between these effects will be seen. But I must premise, that in these experiments, the results are very various at different times with respect to the quantity of effect; I shall therefore relate first such experiments as I set down in a certain part of one day; then mention the differences.
Exp. 30. I held the copper plate on my hand, and laid it upon the zinc plate, which I touched with my finger in this situation, and then carried to the condenser: after twenty repetitions of these alternate contacts, the divergence, then positive, of the gold leaves of the latter, was about half an inch.
Exp. 31. I made the same experiment without touching the zinc plate : the gold leaves struck the sides. I repeated the same process with only ten alternate contacts, and the same quantity of effect was produced as with twenty, when touching the zinc plate.
I must now mention that at other times I have found no difference of effect from either touching or not touching the zinc disc while lying on the copper disc; and that at different times the quantity of effect in both operations was smaller; a point to which I shall return: but these first experiments prove directly, that when zinc is in communication with copper, the former takes from the latter some electric fluid, making other bodies, on its opposite side, share its excess. Thus, after a certain number of repetitions of the alternate contacts, when the upper plate of the condenser is removed, the fluid accumulated on the receiving plate is manifested by the electroscope : and instead of having increased the effect by touching the zinc on the copper, the finger has sometimes, even during that contact, taken off a part of the accumulated fluid. I come to the reverse experiment, made also at a certain part of one day.
Exp. 32. I took the zinc disc on my hand, and, placing the copper disc on it, I touched the latter before carrying it to the condenser, which it made negative. I repeated the experiment without touching the copper disc lying on the zinc disc, and the negative effect was of the same quantity. In general, a greater number of alternate contacts is required to produce the same quantity of negutive effect with copper, than of positive with zinc.
We see however in this experiment the same phenomenon as in the preceding, in this respect, that when zinc and copper are in mutual contact, the former takes some electric Auid from the latter, which, on its opposite side, it shares with other bodies : in the first experiment it shared this fluid with the condenser, while the ground restored it to the copper disc; in the last, zinc communicating that acquisition to the ground, took more fluid from the copper disc, which made the condenser partake of its loss. This is the leading thread with respect to the motion of the electric fluid in the pile, and I shall follow it; but I must first speak more particularly of the anomalies observed in these experiments, which are important.
These differences, sometimes very great, in the quantity of effect of the same operations, surprised me at first; and suspecting some thing amiss in the condenser, I examined it closely, without finding any defect: then at other times, without any change, I found the same effects. At last I remarked that, commonly, the greatest effects in the same day were in a part of the morning, and the smallest towards the evening; and that these effects differed also in intensity on different days. Now this is what I have said above of the spontaneous effects of the pile, which is composed of a succession of the same binary groups of metals; and thus the condenser, when applied to these experiments, is also a meteorologial instrument. And there is a remarkable circumstance in this respect; that often at the same moment there is a great difference between the effects on the condenser of the opposite extremites of a small pile; but sometimes it is the negative side which prevails, and at other times it is the positive. This manifests, that the ground, with which the opposite side of the small pile communicates, in the former case possesses less, and in the latter more of the electric fluid than the ambient air. But this will be one of the subjects of the following paper.
After having shown distinctly what are the electric effects produced by the conjunction of zinc and copper, and in what manner the condenser manifests these effects, I come to their propagation from each group along the pile, on one side negative and on the other positive. The following experiments will show the first steps, which will be easily extended through the whole.
Exp. 33.–Upon a zinc disc, of the same size as the others, I fixed a piece of Dutch gilt paper, the copper side next to the zinc, and the paper outside, held by a little paste all around. This association is to represent one group of the pile, considered for the present only on the copper side with the paper, which, in the pile, separates it from the next group on this side, and more directly from the zinc plate of that group. Holding on my hand the zinc side of the former group, I repeated. on its paper with the insulated zinc disc, the alternate contacts with the condenser; and by twenty repetitions a sensible negative divergence was produced in the electroscope of the latter. The same effect takes place in the pile at the copper
side of every group; it takes, through the paper, some electric fluid from the zinc of the next group; but as zinc must always possess more of this fluid than the copper with which it is con
nected, the zinc of that next group takes more from its own associated
copper, which then through the paper, takes more from the zinc of the following group; which effect goes on increasing, up to the end of the pile on this side. I shall only add to this experiment, that, as the property of a group composed of zinc, copper, and paper on the latter, is to have this side negative, whatever be the metal with which the above alternate contacts are performed, they produce the same negative effect on the condenser.
In order to complete directly the first step here in view, by taking one group as an example, it would be desirable, that by placing a piece of paper on the zinc side of this group, in order to represent the separations of the groups in the pile in that direction, it should manifest directly by the condenser, that zinc yelds some electric fluid to the next group; but I have not been able to obtain such a direct proof, on account of a circumstance which I shall now indicate, as peculiar to that quantity of electric fluid set in motion by the property of the pile. This motion is very slow compared with the motion of an external quantity of electric fluid. The smallest degree of electrification of an insulated body, positive or negative, such as can only affect the gold leaf electroscope, communicated to one side of a pile, is instantly manifested at the other extremity; but it is by no means the same with respect to the electric fluid set in motion by the property of the pile : when the electroscopes diverge equally at the extremities, having touched one of them, which makes the gold leaves fall here, and rise more on the opposite side, it requires a time, often very long, before the same divergence is restored. This is the reason why I was obliged to fix twenty seconds for the duration of the contacts of my piles of twenty groups with the condenser, in order to be certain that the maximum of effect was produced ; it requires more or less time, according either to their nature, or to the extremity applied to the condenser ; but I found, that the slowest was produced in twenty seconds, which made me fix that time.
This will explain the case abovementioned, that there is no sensible effect on the condenser by any number of alternate contacts of an insulated body, with the paper laid on the zinc side of one group; and even when the groups are multiplied to increase the effect, it requires, to make it sensible, a prolongation of the contact on the little pile, in order to give time for the effect to be propagated ; as will be seen by the following experiment.
Exp. 34. Made with a portion of my pile of ten groups of 1.6 inch diameter, successively tried by each of their extremities, the opposite one being placed on the movable pillar of my condenser, and the alternate contacts being made with a small insulated disc of the same diameter.
1. No number of alternate contacts, between either of the extremities of the little pile and the condenser, produced any sensible effect on the latter when rapidly made, or in the manner they are executed between the two naked discs, zinc and copper.
2. On repeating the experiment, with the zinc side on the pillar,
and lengthening the time of the contact on the copper side up to four seconds, I found, that by twenty such contacts the electroscope was affected negatively to a certain measurable quantity.
3. Inverting the little pile, I had no sensible effect by these contacts of four seconds on the zinc side, and it was necessary to lengthen the time to eight seconds; but in order to produce on the electroscope a positive effect equal to the negative of the preceding trial, I was obliged to make forty contacts, on account of the dissipation of the effects on the condenser in each interval of time. This experiment shows however, sufficiently, that the zinc side of each group yields, through the paper, to the next group, some of the electric fluid that it takes from the copper with which it is associated.
We have from these experiments all the elementary principles necessary for the motion of the electric fluid in the pile, and they are the following :1. In each binary group, the zinc plate takes some electric fluid from its associate the copper; the latter in my new pile, being the coppered side of the Dutch gilt paper. 2. In each group also, zinc communicates, through the paper, some of its excess of fluid to the copper of the next group on its side. 3. In each group again the copper takes, through the paper from the zinc of the next group on its side, some of the fluid that it has lost to its associated zinc. The same effects taking place in every group, with the next on both sides, along the whole pile, these effects are successively added to those that the respective next groups have already undergone according to their place; and thus the negative state goes on increasing from one end to the other of the pile, toward what is called the copper extremity ; and the positive state is increasing toward the zinc extremity.
These effects may be represented by numbers ; though, from the great variations in the quantity at different times, and the imperfection of the electroscopes, these numbers remain undetermined: I shall express them in a pile of eleven groups, indicating by A the zinc side, and by B the copper side. The two following series represent the progress of negative and positive effects above mentioned, which, combining in each successive group from A to B, constitute, according to circumstances, the three different states of the pile. A
+ 8 3
+ 6 5
+ 1 -10
In the insulated pile, when the divergence of the electroscopes is equal on both sides, positive at A, and negative at B, the state of each successive group is the sum of the corresponding numbers of the above series, as expressed in the 1st of the following Tables. When B is placed in communication with the ground, the losses of all the copper plates being repaired by the latter, all the acquisitions of the zinc plates subsist without diminution, which requires the quantity ten, to be added to each number of Table I, as expressed in Table II. When A communicates with the ground, all the acquisitions of the zinc plates being carried into the latter, the losses of the copper plates remain uncompensated, and the same quantity, ten, is to be substracted from all the numbers of Table I, as expressed in Table III. TABLE I. TABLE II.
B Now this synthesis of the above fundamental experiments is the real fact; as will be seen from direct experiments in the following paper, of which I shall give here only the general results.
For these experiments I use a horizontal pile, which I have called column ; with a gold leaf electroscope at each extremity; and I have also a detached electroscope, which may be applied and observed at every point of the column. The following are the observed phenomena :-1. When the state of the ambient air is such, that in the insulated column the divergence is equal at both extremities, the middle point in its length is zero as represented in Table I, in which, as well as in the two others, the terms are to be considered only as equidistant points, whatever be the number of the groups. 2 When B communicates with the ground, the first plate only at this extremity is zero, and the positive state is gradually increasing towards A: the middle point is plus, of the same quantity (sensibly) as it was at A .in Table I, and the divergence plus is doubled at A; a state represented in Table II. 3. When A communicates with the ground, all the effects are reversed : the first plate only at A is zero, and the negative state is gradually increasing towards B: the middle point