Additional experiments and observations on the application of electrical combinations to the preservation of the copper sheathing of ships, and to other purposes

Abstract

Since his former communication the President has had an opportunity of prosecuting his researches upon the above subjects, upon an extended scale, and with results perfectly conclusive and satisfactory. He found that sheets of copper defended by from one 100th to one 150th part of zinc or iron, exposed for many weeks to the full flow of the tide in Portsmouth harbour, suffered no corrosion, and that even one 1000th part of cast iron exerted great protecting influence. Boats and the sides of ships protected in this way were also similarly preserved. Of the different protecting metals cast iron is most convenient, and the plumbaginous substance formed upon it does not impede the electrical action. The President formerly anticipated the deposition of earthy substances upon the negative copper, and this he now found to take place upon sheets of copper exposed about four months to seawater, and defended by from one 50th to one 80th their surface of zinc and iron. They became coated with carbonate of lime and magnesia; but this effect is easily prevented by duly diminishing the proportion of the protecting metal, so as to prevent the excess of negative power in the copper, which then remains bright and clean. The author observes, that many singular facts have occurred in the course of his researches, some of which bear upon general science. Weak solutions of salt act strongly upon copper, but strong ones do not affect it, apparently because they contain little air, the oxygen of which seems necessary to give the electro-positive power to these menstrua; upon the same principle, alkaline solutions and lime-water prevent the action of sea-water on copper, having in themselves the positive electrical energy which renders the copper negative.