Harper's Electricity Book for Boys

by Joseph H. Adams

Another manner in which the bottom of the tank can be attached is shown in [Fig. 5], which is a view of the tank sides turned bottom up. A rabbet is cut from the lower edges of the sides and ends, before they are screwed together, and a bottom is fashioned of such shape as to accurately fit in the lap formed by the rabbet. This rabbet and the outer edge of the bottom plank should be well smeared with white-lead, and all put together at the same time, driving the screws into the edge of the bottom plank, through the lower edges of the sides and bottom, and also through the bottom board into the lower edges of the sides and ends ([Fig. 6]).

Still another and stronger way in which to make a tank for a large bath is to cut the planks as shown at [Fig. 7]. The sides are then bolted together, locking the ends and bottom, so that they cannot warp or get away. The bolts are of three-eighth-inch round iron-rod, threaded at both ends and provided with nuts. Large washers are placed against the wood and under the nuts, so that when the nuts are screwed on tightly they will not tear the wood, but will bear on the washers. The points are all to be well smeared with white-lead or acid-proof cement (see Formulæ) before the parts are put together and bolted, so as to avoid any possibility of leakage. ([Fig. 8] shows the completed tank.)

TANK FOR ELECTRO-PLATING

Now obtain two copper rods long enough to span the tank, with an inch or two projecting beyond the tank at either side. At one end of these attach binding-posts, to which the wires from a battery can be connected, leaving the opposite ends free, as shown at [Fig. 9] (see [page 275]). Anodes, or pure soft copper plates, are hung on the positive rod, while on the negative one the objects to be plated, or kathodes, are suspended on fine copper wires just heavy enough to properly conduct the current. The positive wire leads from the carbon, or copper pole, of the battery, while the negative one is connected with the zinc. The anodes are plates of soft sheet or cast copper, and should be as nearly pure as possible for electrolytic work; but if they are to be re-deposited, to free them from impurities, they may be in thin ingot form, just as the copper comes from the mines.

The general principle of electro-refining of copper is very simple. A cast plate of the crude copper is hung from the positive pole in a bath of sulphate of copper, made by dissolving all the sulphate of copper, or bluestone, that the water will take up. Drop a few lumps on the bottom of the tank to supply any deficiency, then add an ounce of sulphuric acid to each gallon of liquid, to make it more active and a better conductor.

The crude copper plate is to be the leading-in pole for the current, while a thin sheet of pure copper, no thicker than tissue-paper, is suspended from the opposite rod for the leading-out pole; or in place of the thin sheet, some copper wires may be suspended from the rod. The electrodes—that is, the copper plate and the thin sheet or wires—are placed close together, so that the current may pass freely and not cause internal resistance in the battery. The electric current, in its passage from the crude copper plate to the pure copper sheet or wires, decomposes the sulphate of copper solution and causes it to deposit its metallic copper on the sheet or wires; and at the same time it takes from the crude copper a like portion of metallic copper and converts it into chemical copper. The electric current really takes the copper from the solution and adds it to the pure copper sheet, while the remaining constituents of the decomposed solution help themselves to some copper from the crude plate. In this way the crude copper diminishes and the pure copper sheet increases in size, the impurities as well as the salts of other metals being precipitated to the bottom of the tank, or mingled with the solution, which must be purified or replaced from time to time by fresh solution. This is the process of copper-plating, and any metal object may be properly cleansed and coated with copper by suspending it in the bath and running the current through it.

When the refining process is employed, any metal will answer as a depository for the copper, but as the intention is to produce a pure copper plate which can be melted and cast into ingots, it is of course necessary to have the original kathode of the same metal; otherwise an impure mixture will be the result. If, for example, a piece of cast-iron be used upon which to deposit the copper, then the iron will be enclosed in a deposit of pure copper; in other words, the result will be a heavily copper-plated piece of iron, and the smelting process will bring about a fusion of the two metals. It is not necessary to have absolutely pure copper for the anodes when copper-plating or electrotyping; but the purer the copper the less the solution is fouled, and it will not require replenishing so often.

An object intended to receive a plating of copper need not be of metal at all; it may be of any material, so long as it possesses a conducting surface. A mold or a cast made of any plastic material, such as wax or cement, may have its surface made conductive by the application of graphite, finely pulverized carbon, or metal dusts held on by some medium not soluble in water. The wax molds, or impressions of type and cuts, are dusted with plumbago, and then suspended in the copper solution. A wire from the negative pole is connected so as to come in contact with the plumbago, and the copper deposit immediately begins to form on the face of the wax. When the film of copper has become heavy enough, the mold is drawn out of the solution, and the thin shell of metal removed from the wax and cut apart, so that each shell is separated from its neighbor and freed from marginal scraps. Flowers, leaves, laces, and various other objects can be given a coat of copper by thus preparing their surfaces, and some most beautiful effects may be secured by copper-coating roses; then placing them for a short time in a gold bath, and afterwards chemically treating the surface plating so as to imitate Roman, Tuscan, or ormolu gold, in bright or antique finish. Coins, medallions, bas-reliefs, medals, and various other things are reproduced by the electro-plating process, and their surfaces finished in gold, silver, bronze, or other effects. Years ago this was not possible, because the old method was to make a fac-simile cast in metal of the object desired, and then chase or refinish the surface. This was a costly and tedious task. When Brugnalelli, an Italian electrician, electro-gilded two silver coins in 1805, he laid the foundation for the modern process, but it did not come into general use until about 1839, when electro-plating and the electro-depositing of metals was begun on a practical scale. Before the invention of the dynamos for generating current, batteries had to be employed, and this made the process somewhat more expensive than the present method. Our boy amateurs, however, will have to be content with the battery system, since they are not supposed to have access to direct-current power, such as is used for arc or street lighting.