The copper sulphate contained in the balloon is dissolved in the water around it, and as this solution is denser than water it falls into the porous cells through one of the notches of the cork, while an equal quantity of purer and lighter water ascends through the other notch, and so on, producing a circuit of denser liquor falling by one notch, and of lighter liquor rising by the other. The solution of copper sulphate is decomposed in the porous cell; the sulphuric acid passes through the cell by outward pressure and acts upon the zinc, and at the same time the copper becomes deposited upon the copper ribbon connected with the zinc of the former element. In order that this battery may work regularly for 6-7 months, it is sufficient to replace the evaporated water. The balloon ought to contain at least 2 lb. of copper sulphate, and the zinc to be about 7 in. high, and 4-4 1/2 in. diameter. The zinc may be amalgamated, in which case the action is a little slow at the start, but more regular afterwards. The copper ribbon receives all the metal of the decomposed sulphate, and it sometimes happens that part of the copper becomes deposited upon the porous cell, which must then be cleaned in aquafoitis.

When all the copper sulphate is used up, the balloons are filled with a fresh quantity of crystals and new copper ribbons are inserted to take the place of those rendered too voluminous. If it be desired to start the battery with a balloon immediately, add a small quantity of sulphuric acid, or of common salt, to the water in which the zinc is placed.

For the requirements of a small plating shop the Daniell cell is perhaps the best. Two cells made as fellows will do very well for small work: Procure two stoneware jars large enough to hold about 2 gal. each; turn up two pieces of light sheet copper, so as to form cylinders, a little smaller than the internal diameter and about the height of the jars. In the centre of each of these cylinders stand a porous cell (the tops of these cells to the depth of about 1 in. should be soaked in hot paraffin wax to prevent the creeping action of the .salts in solution). In each porous cell place a cast zinc rod about 1 in. diameter; lastly, solder terminals to each zinc and copper. The charge for this battery consists of a saturated solution of copper sulphate, (-lightly acidulated with sulphuric acid, in outer stoneware cell, and sulphuric acid I part, water 12 to 15 parts, in porous cell. If the zincs are kept well amalgamated, the sulphate solution saturated, and both solutions at the same level, this battery will continue to give a steady current for a considerable time.

All wires used for connections should be large, not less than 14 B.W.G., so that the external resistance is kept as low as possible.

Grove's

This battery has a platinum foil which plunges into the nitric acid, and replaces the prism of carbon. This foil is supported by a small brass stand, fixed itselftoa round band resting upon a rim on top of the exterior vase. A binding screw is soldered to the stand when connection is to be made with the copper ribbon of the preceding zinc. The several elements of batteries are united together in the manner already mentioned, the zinc to the platinum of the next element, and so on. The disadvantage of this battery is its great cost, due to the platinum employed; it has been proposed to substitute aluminium, but still the battery is an expensive one.

Grenet's

A solution of 100 parts water, 10 of potash bichromate, and 10 or sulphuric acid in the porous cell, replaces the nitric acid employed by Grove and Bunsen. This battery does not emit acid fumes, but the carbon is rapidly incrusted with chromium oxide, which arrests the galvanic current.

Marie-Davy

Slightly damp mercury sulphate replaces the nitric acid in the porous cell. The working expenses of this battery are very high, and it is used only in the telegraphic service, where the Daniell battery with balloons is not preferred.

Smee's

This battery is very simple in construction. It is composed of a thick wooden frame open at the top, with three internal parallel grooves which run the height of the two opposite sides. The middle groove receives a movable plate of silver, platinum, gold, or copper which has been strongly gilt, silvered, or platinised; its surfaces must be rough or with a dead lustre. Two plates of strongly amalgamated zinc are run down the other two grooves. The plates of zinc must be near to, but not in contact with, the central one, and are connected by a wire or metallic band. The positive wire starts from the middle plate, and the negative from the zinc, and the whole apparatus is immersed in a solution containing common salt or 1/10 of sulphuric acid. Several elements may be united together by connecting the zinc of the first with the middle plate of the second. Or the cell may be made of gutta-percha, with a plate of carbon to replace the plate of silver, or of platinised copper. The two other grooves receive two plates of amalgamated zinc with one of the upper corners cut away. A double binding screw, for the positive wire, is fixed upon the plate of carbon where the two zinc corners have been cut off, and another large binding screw unites the two zinc plates, and carries the negative wire.

Fill the cell with water saturated with common salt, or acidulated with 1/10 of sulphuric acid.

Watt's

In a stoneware jar holding about 4 gal., place a cylinder of thin sheet copper, dipping into water acidulated with 2 lb. sulphuric acid and 1 oz. nitric acid. A solid zinc cylinder is put into the porous cell, which is filled with a concentrated solution of common salt, to which a few drops of hydrochloric acid have been added.

Vats, Etc

For small work, earthenware pans, white glazed inside, make very good vats. The size depends to a great extent on the class of work likely to be undertaken, but one holding 4-5 gal. is a very useful size. If possible, the pan should be about the same in depth as it is in diameter. A large bell-glass or fish globe serves well.