Abstract
AFTER the President's address on Thursday morning, Lord Kelvin opened the proceedings in Section A with an account of some preliminary experiments made by himself and Mr. Maclean on the electrification of air by the subtraction of water from it. The subject is one in which Lord Kelvin has been for many years interested, and he commenced experimenting on it as far back as 1868. The nature of the results now obtained was illustrated by his insisting that the proper title of the paper was “Preliminary experiments to find if subtraction of water from air electrifies it” (and not as in the Journal—“Experiments proving the electrification of air”). In the present investigation a large U-tube was used. One branch of this was filled with pumice-stone soaked in sulphuric acid; the other was simply varnished inside and out. By means of a platinum wire touching the pumice, connection was made with a quadrant electrometer. A metal cylinder screened the tube from external influence. Air from an ordinary blow-pipe bellows was blown through the tube steadily for an hour; and the electrometer showed an electrification rising gradually to about nine volts positive. This shows that the passage of the air through the tube gave positive electricity to the acid, and therefore sent away the dried air electrified negatively. No such effect was observed when the pumice was moistened with water instead of sulphuric acid. The experiments are to be repeated with precautions to prevent any babbling of the air through liquid in the tube; for it was observed that the strong positive electrification of the tube (when acid or calcium chloride was used) seemed to commence suddenly as soon as a gurgling sound, due to bubbling through free liquid, began to be heard. The authors have reversed the conditions, and have first dried air by passing it over sulphuric pumice, and then passed it through a tube containing moistened pumice. The tube became negatively electrified, but this may have been due to the negative electrification of the dry entering air. This experiment is to be repeated with dried and dis-electrified air. Lord Kelvin also described certain preliminary experiments made by himself and Mr. Gait with the object of comparing the discharge of a Leyden jar through different branches of a divided channel. The metallic part of the discharge channel was divided between two wires of conducting metal, each consisting in part of a test-wire. Each of the two test-wires consisted of 51 cm. of platinum wire of 0˙006 cm. diameter and 12 ohms resistance stretched in a glass tube. One end was fixed to a solid brass mounting, and the other was attached to a fine spring carrying a light arm for multiplying the motion. The testing effect was the heat developed in the test-wire by the discharge, as shown by the elongation, the amount of which was measured by a tracing on sooted paper carried by a drum. The wires to be tested were generally of the same length. When they were of the same material but of different diameters, the testing elongation showed, as might be expected, that the test-wire in the branch containing the thicker wire was more heated than the other. With wires of various nonmagnetic materials, of the same resistances but different lengths and diameters, the testing elongations were very nearly equal. In one experiment two equal copper wires were used, but one of them was coiled into a helix; the testing elongation in this branch was less than half of that in the straight branch. Lastly an iron wire was compared with a platinoid wire of equal resistance but greater diameter. Tne heating effect in the platinoid branch was nearly one-and-a-half times as great as in the iron branch. This is interesting in relation to Lodge's experiments on alternative paths, which were not decisive in showing any general superiority of copper over iron of the same steady ohmic resistance, but even showed a seeming superiority of the iron for efficiency in the discharge of a Leyden jar.
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Physics at the British Association. Nature 50, 406–409 (1894). https://doi.org/10.1038/050406a0
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DOI: https://doi.org/10.1038/050406a0
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