Further researches on the preservation of metals by electro-chemical means

Abstract

After adverting to the general details respecting the protection of the copper sheathing of ships, contained in his former papers, the President proceeds, in the present communication, to consider the circumstances under which various substances are deposited upon the protecting copper, and their general influence upon its wear, more especially in regard to ships in motion. For this purpose, he availed himself of the use of a steam boat, employed on an expedition to ascertain some points of longitude in the North Seas, and his inquiries lead to the inference that motion does not affect the nature of the limits and quantity of the protecting metal ; and that, independently of the chemical, there is likewise a mechanical wear of the copper in sailing. In examining the results of some of the experiments upon the effects of single masses of protecting metal on the sheeting, the author observed, that in some cases the corrosion seemed to increase with the distance from the protector. It became, therefore, necessary to investigate this circumstance, and to ascertain the extent of the diminution of electrical action in instances of imperfect or irregular conducting surfaces. Sir Humphry details several experiments in illustration of this inquiry, which prove that any diminution of protecting effect at a distance does not depend upon the nature of the metallic, but of the imperfect or fluid conductor. His experiments upon perfect and imperfect conductors led him to another inquiry, important in its practical relations, respecting the nature of the contact between the copper and the preserving metal. He found the protecting action prevented by the thinnest stratum of air, or the finest leaf of talc or dry paper ; but the ordinary coating of rust, or a thin piece of moistened paper, did not impair it.