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.
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.
To charge this battery, add five fluid ounces of sulphuric acid to three pints of cold water, pouring the acid slowly into the water and stirring it at the same time with a glass or carbon rod. When this becomes cold, after standing a few hours, add six ounces of finely pulverized bichromate of potash. Mix this thoroughly, and pour some of the solution into the glass cell until it is three-fourths full; then it will be ready to receive the carbons and zinc. When arranging the wood-clamps on the carbon and zinc plates it would be well to make two of the clamps longer than the others so that they will extend out far enough to rest on the top edge of the jar. To keep them in position at the middle of the jar, notches should be cut at the underside of these clamps, so that they will fit down over the edge of the jar. Any number of these cells may be connected together to obtain the desired amount of current, or electro-motive force.
Other batteries suitable for electro-plating are the Edison primary, Taylor, Fuller, Daniell, gravity, Groves, and Merdingers. All of these may be purchased at large electrical equipment or supply houses.
The Cleansing Process
One of the most important operations of the plating process is to properly cleanse the articles to be plated before they are placed in the bath. When once cleaned the surfaces of these objects must not be touched with the fingers, or any dusty or greasy object; otherwise the electro-deposited metal will not hold on the surface, but will peel off, in time, or blister. A very small trace of foreign matter is sufficient to prevent the deposit from adhering to the surface to be plated; therefore, great care must be taken to eliminate all trace of anything that would interfere with the perfect transmission of metallic molecules to the prepared surfaces. Acids are chiefly employed to remove foreign matter from new metallic surfaces; and for copper, brass, iron, zinc, gold, and silver a table is given on [page 281] which will show the right proportion of acids to water in order to cleanse the various metals. In the following scale the numerals stand for parts. For example: the first one means 100 parts water, 50 parts nitric acid, 100 parts sulphuric acid, and 2 parts hydrochloric acid—making in all 252 parts. These can be measured in a glass graduate.