The case of contact being thus completely ana lysed, let us examine what takes place when the two spheres separated from their contact are removed to a certain distance from each other.
In this case, a very remarkable phenomenon, dis covered by Coulomb, is developed, and of which that philosopher determined the measure by his electric balance. We have seen that, during the contact, the electricity is the nine on both the spheres. To fix our ideas, suppose it to be the vitreous. As yet its intensity is nothing, as we have seen, at the point of contact; but the instant we separate the two spheres, if their dimensions are unequal, this state of things ceases. A part of the combined electricities of the mall sphere is decomposed, and what is of a contrary nature to that of the large sphere, namely, the resin ous in our example, in, and near the point where the contact took place. This effect diminishes in proportion as we remove the spheres from each other, and vanishes altogether at a certain distance, which depends on the proportion of their radii. At that distance, the point of the small sphere, where the contact was made, returns to the state in which it was during the contact itself, that is, it has no more any species of electricity. From this position, if we still increase the distance, the electricity remains of the same nature over all the extent of the small sphere, and of the same nature also as it was du ring the contact These changes are always pe culiar to the smallest of the two spheres, whatever be the quantity of electricity which has been previ ously communicated to either. As to the larger of the two spheres, the electricity is always, and every where, of the same nature as at the moment of con tact To observe these phenomena, we place the two spheres upon solid supports of some insulting sub stance, and of such a magnitude that their centres may be in a horizontal plane, as represented inthe figure.(5.) After having placed 1.11C111111 031118A:411,1311 electrified them to gether, we remove them to a small dis tance, A a, from each other. We then carry the proof plane first to A upon the large sphere, to the point where the contact was made ; and by trying, not with the elec trical balance, but with a very sensible electroscope, the electricity which it carries off, we observe that it is always of the same nature with that upon the rest of the surface ABDE. Making then the same trial
at the corresponding point a, we find that the electri city in this point is, first, of a nature contrary to that of the large sphere; then nothing when the distance becomes a little greater; then of the same nature when the distance becomes still greater; and in this man ner, by gradually augmenting the distance, we see the phenomena pass through all the periods which have been already pointed out by the theory. These trials must be all made with the most sensible elec troscope, in order to perceive the feeblest signs of either electricity. , In one of Coulomb's experiments, the large globe was twelve inches in diameter, and the small one eight. As long as the distance A a was less than an inch, the point a gave signs of a contrary elec tricity to that of the large lobe. When the dis tance became equal to an inch, the electricity of this point became equal to nothing, as at the in stant of contact; and, lastly, at every distance be yond this, it became of the same nature with that of the other sphere. The large globe remaining the same as in the preceding experiment, Coulomb gave to the small globe a diameter of only four inches ; then the two electricities continued of an opposite nature to the distance of two inches. When this diameter was only two inches and under, the o sition was kept up to the distance of two s five lines, but no farther.
The comparision of these experiments shows, that the distance A a, where the opposition of the elec tricity ceases, diminishes in proportion as the two globes approach to an equality, and becomes no thing when they become equal. This is a circum stance which is also confirmed by the theory, which equally indicates all the other details of the pheno menon, the relation between the radii of the two globes being given.