In copper-plating this is easy to determine without any regard to wires, because if the wires are misconnected there will be no deposit, and the kathode will turn a dark color. If everything is all right a slight rose-colored blush of copper will appear at once on the kathode. Too little current will make the process a long and tedious one, while too much current will deposit a brown mud on the kathode, which will have to be washed off or removed and the article thoroughly cleansed before a new action is allowed to take place. With a series of cells it is an easy matter to properly govern the current by cutting out some of the cells or by using resistance-coils (see chapter vii. on Electrical Resistance). Cells and batteries for electro-plating may be made or purchased, and primary batteries should be used. The use of the secondary or storage-battery is not necessary for plating purposes, since no great volume of current is needed, and it can be generated in a battery of cells while the work is going on. One of the best primary batteries is the Benson cell, shown in connection with the plating-bath, and also in Fig. 11. It consists of an outer glass jar (G J), which contains a cylinder of amalgamated zinc (Z +, or positive) covered with diluted sulphuric acid--one part acid to three parts water. An inner porous cup (P C) contains concentrated nitric acid, into which the carbon (C -, or negative) is plunged. The liquid in the inner cup and glass cell should be at the same level. [Illustration: FIG. 10 FIG. 11 FIG. 12 THE BENSON CELL PRIMARY BATTERY] There is no polarizing in this cell, for the hydrogen liberated at the zinc plate, in passing through the nitric acid on its way to the carbon-pole, decomposes the nitric acid and is itself oxidized. A cell with a glass jar six inches in diameter and eight inches high will develop about two volts of electro-motive force; and as its internal resistance is very low it will furnish a steady current for several hours. Any number of these cells may be made and connected in series; but when not in use it would be well to remove and wash the zincs. Any bichromate battery will answer very well for plating, the Grenet being an especially good one. A well-amalgamated zinc plate forms one pole, and a pair of carbon plates, one on each side of the zinc and joined at the top, make up the other pole. When not in use the entire plunge part should be removed from the bichromate solution, rinsed off in water, and laid across the top of the jar, ready for its next employment. The zinc and carbons must be joined together so that they are well insulated, and with no chance of the zinc coming into contact with the carbons. This may be done with four pieces of hard-wood soaked in hot paraffine and then locked together with stove-bolts and nuts, as shown at Fig. 12. Holes must be made in the top corners of the carbons and zinc, and with small bolts and nuts the connecting wires can be made fast.