ELECTRO-METALLIIRGY.
The deposition of metals upon prepared surfaces from solutions of their salts, by electrical action, is the object of the art of electro-metallurgy. So far as can be ascertained, it has only been applied in a commercial form during the present century. Two great branches of the many processes included under this general name ere extensively practised ; in the one a thick non adherent deposit, generally of copper, is required, as in reproducing metallic facsimiles of wood engravings, or electrotypes as they are termed, small statues, and the like ; whilst in the other a thin adherent deposit is sought, either for the protection or ornament of the article to which it is applied. In practising electro-metallurgical processes, the principles of both chemical and electrical action are largely intermingled end brought into play, and therefore some little knowledge of these principles ie essential for their intelligent conduct. The varying powers of conducting and insulation which various bodies possess has been repeatedly made a matter of investigation. Con ductors are those substances which freely allow of the passage of electricity ; whilst non-conductors, or dielectrics as they are often called, resist its passage. The best conductors are the metals, graphite, water, alcohol, dilute acids, oils, metallic oxides, and some few others ; whilst most of the gums, silica in every form, including glass, ebonite, shellac, indiarubber, guttapercha, sulphur, wax, wool, hair, dry paper, are dielectrics ; practically the insulating substances usually employed are either guttapercha, glass or indiarubher, cotton or silk ; thus wires for conducting electricity are covered with either of these substances, whilst glass is oommonly used for supports which require to be insulated, and also for enclosing the wires on which many articles are suspended in solution.
Taking silver as equal to 100, Matthiessen has stated that the relative conducting powers of pure metals are The slightest admixture of alloy will materially influence the conducting effect of a metal ; whilst many foreign substances which enter into the composition of metals, if not eliminated, so far interfere with their conductivity as to exclude their use for electrical purposes. Of the above,
copper is the metal most commonly used for conducting; it is very flexible, easily obtained, and, whilst not being readily oxidized, can be procured with sufficient pureness to render it available for most purposes. Iron wire is rarely used for electro-metallurgical purposes, in consequence of its high resistance.
Batteries are only employed in small work, as operations of magnitude are usually conducted with the aid of magneto-electro machines. The batteries most in favour are Wollaston's, Smee's, Daniell's, Bunsen's, and Grove's. Battery selection and management are matters involving consider able experience ; it may, however, be accepted, that Wollaston's is the most suitable one in cases where the resistance is not great, and where a large quantity of thetrieity and long-continued action, as in depositing copper and silver, are required ; its action after commencement is uniform, and large plates with considerable bulks of exciting liquid may be used. Smee's is available for similar work on a smaller scale, as when of large size it is expensive. Daniell's is best when the resistance is great and a very uniform current is necessary. Grove's and Bunsen's are preferable where the resistance is still greater, and occasional currents of considerable electromotive force, but not of long continuance, are necessary, as in gilding, nickel plating, and in brassing or coppering in cyanide solutions on iron.
Magneto-electric machines, such as those of Gramme, Wilde, Siemens and Alteneck, Weston, Elmore, and others, are extensively used in large establishments, since it has been conclusively proved that the magneto-electric machine possesses innumerable advantages over the battery for plating purposes.