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.) [Illustration: FIG. 1 FIG. 2 FIG. 3 FIG. 4 FIG. 5 FIG. 6 FIG. 7 FIG. 8 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.