A History of Science-2 [101]
pack-thread by pieces of string looped over nails driven into beams for that purpose. But when thus suspended he found that the ivory ball no longer excited the leaf-brass, and he guessed correctly that the explanation of this lay in the fact that "when the electric virtue came to the loop that was suspended on the beam it went up the same to the beam," none of it reaching the ball. As we shall see from what follows, however, Gray had not as yet determined that certain substances will conduct electricity while others will not. But by a lucky accident he made the discovery that silk, for example, was a poor conductor, and could be turned to account in insulating the conducting-cord. A certain Mr. Wheler had become much interested in the old pensioner and his work, and, as a guest at the Wheler house, Gray had been repeating some of his former experiments with the fishing-rod, line, and ivory ball. He had finally exhausted the heights from which these experiments could be made by climbing to the clock-tower and exciting bits of leaf-brass on the ground below. "As we had no greater heights here," he says, "Mr. Wheler was desirous to try whether we could not carry the electric virtue horizontally. I then told him of the attempt I had made with that design, but without success, telling him the method and materials made use of, as mentioned above. He then proposed a silk line to support the line by which the electric virtue was to pass. I told him it might do better upon account of its smallness; so that there would be less virtue carried from the line of communication. "The first experiment was made in the matted gallery, July 2, 1729, about ten in the morning. About four feet from the end of the gallery there was a cross line that was fixed by its ends to each side of the gallery by two nails; the middle part of the line was silk, the rest at each end pack-thread; then the line to which the ivory ball was hung and by which the electric virtue was to be conveyed to it from the tube, being eighty and one-half feet in length, was laid on the cross silk line, so that the ball hung about nine feet below it. Then the other end of the line was by a loop suspended on the glass cane, and the leaf-brass held under the ball on a piece of white paper; when, the tube being rubbed, the ball attracted the leaf-brass, and kept it suspended on it for some time." This experiment succeeded so well that the string was lengthened until it was some two hundred and ninety-three feet long; and still the attractive force continued, apparently as strong as ever. On lengthening the string still more, however, the extra weight proved too much for the strength of the silk suspending-thread. "Upon this," says Gray, "having brought with me both brass and iron wire, instead of the silk we put up small iron wire; but this was too weak to bear the weight of the line. We then took brass wire of a somewhat larger size than that of iron. This supported our line of communication; but though the tube was well rubbed, yet there was not the least motion or attraction given by the ball, neither with the great tube, which we made use of when we found the small solid cane to be ineffectual; by which we were now convinced that the success we had before depended upon the lines that supported the line of communication being silk, and not upon their being small, as before trial I had imagined it might be; the same effect happening here as it did when the line that is to convey the electric virtue is supported by pack-thread." Soon after this Gray and his host suspended a pack-thread six hundred and sixty-six feet long on poles across a field, these poles being slightly inclined so that the thread could be suspended from the top by small silk cords, thus securing the necessary insulation. This pack-thread line, suspended upon poles along which Gray was able to transmit the electricity, is very suggestive of the modern telegraph, but the idea of signalling or making use of it for communicating in any way seems not to have occurred to any one at that time. Even the successors