Numerous other experiments were performed, and with results equally conclusive of the truth of the theory which had suggested them.
There was however one point which still remained for enquiry. As the ocean may be considered in its relation to the quantity of copper in a ship, as an infinitely extended conductor, it became necessary to ascertain whether that circumstance would influence the results. For this purpose, he placed two very fine copper wires, one undefended, the other defended by a particle of zinc, in a very large vessel of sea-water, which water might be considered to bear the same relation to so minute a portion of metal, as the sea to the metallic sheathing of a ship. The result was perfectly satisfactory. The defended copper underwent no change; the undefended tarnished, and deposited a green powder.[93]
Davy having thus satisfied his own mind as to the truth of his views, communicated to Government, in January 1824, the important fact of his having discovered a remedy for the evil of which they had complained; and that the corrosion of the copper sheathing of his Majesty's ships might be prevented by rendering the copper electro-positive, by means of the contact of tin, zinc, lead, iron, or any other easily oxidable metal; and that he was prepared to carry his plan into effect.
A proposition from a philosopher of such known science, and upon a subject of such great importance to the navigation and commerce of the country, immediately obtained all the attention it deserved; and an order was made that the plan of protection should, under the superintendence of Sir H. Davy, be forthwith tried upon the bottom of a sailing cutter.
To give to his discovery farther publicity, Sir Humphry requested that three models of ships might be exhibited in the spacious hall of the Navy Office in Somerset House; the copper of one of which he proposed should be protected by bands of zinc, that of another by plates of wrought iron soldered on the sheathing, while the third should have its copper exposed without any protection whatever.
These models were floated in sea-water for several months; and the experiment fully confirmed the results he had previously obtained in his laboratory. The models were from time to time examined by persons of the highest scientific character, as well as by others of great naval celebrity; and so alluring was the theory, and so conclusive the experiments, that, instead of waiting the result of the slow but more certain ordeal to which the plan had been submitted, it was immediately put into extensive practice, both in the Government service and on the bottoms of ships belonging to private individuals.
To those the least acquainted with the principles of Voltaic action, it was only necessary to state the proposition, in order to command their assent to its truth. The utility of the plan therefore was never questioned, but the claims of Davy to the originality of the invention were doomed to meet with immediate opposition.[94]
The correctness of the principle having been established, it became, in the next place, necessary to determine the most eligible metal to be used for protection; the proportion which it must bear to the surface of the copper-sheathing below the waterline; the form least likely to offer resistance to the sea, and to impede the sailing of the vessel; and lastly, its most convenient position on the ship's bottom. To ascertain these several points, Lord Melville and the Lords of the Admiralty desired the Commissioners of the Navy Board, and of the Dock-yards, to afford Sir Humphry every assistance and facility for prosecuting the necessary experiments; and he accordingly made many very extensive trials, not only on copper sheets which were immersed in the sea, but also on the bottoms of a considerable number of boats which had been coppered for that purpose, and exposed to the flow of the tide in Portsmouth harbour; upon which occasions he varied the nature as well as the proportions of the protecting metal. The results were communicated to the Royal Society, and they constituted the materials for his second memoir on the subject.
"When the metallic protector was from 1/20 to 1/110 parts of its surface, there was no corrosion nor decay of the copper; with smaller quantities, such as from 1/200 to 1/400, the copper underwent a loss of weight, which was greater in proportion as the protector was smaller; and, as a proof of the universality of the principle, it was found that even 1/1000 part of cast iron saved a certain proportion of the copper.
"The sheeting of boats and ships, protected by the contact of zinc, or cast and malleable iron in different proportions, compared with those of similar boats and sides of ships unprotected, exhibited bright surfaces; whilst the unprotected copper underwent rapid corrosion, becoming first red, then green, and losing a part of its substance in scales. Fortunately, in the course of these experiments, it was proved that cast iron, the substance which is cheapest and most easily procured, is likewise most fitted for the protection of the copper. It lasts longer than malleable iron, or zinc; and the plumbaginous substance which is left by the action of sea-water upon it, retains the original form of the iron, and does not impede the electrical action of the remaining metal."