The deposition of a metal from a solution of its salt is very easily shown in the case of copper.  In fact, we have already seen that in the Daniell cell the current decomposes a solution of sulphate of copper and deposits the pure metal on the copper plate.  If we simply make a solution of blue vitriol in a glass beaker and dip the wires from a voltaic cell into it, we shall find the wire from the negative pole become freshly coated with particles of new copper.  The sulphate has been broken up, and the liberated metal, being positive, gathers on the negative electrode.  Moreover, if we examine the positive electrode we shall find it slightly eaten away, because the sulphuric acid set free from the sulphate has combined with the particles of that wire to make new sulphate.  Thus the copper is deposited on one electrode, namely, the cathode, by which the current leaves the bath, and at the expense of the other electrode, that is to say, the anode, by which the current enters the bath.

The fact that the weight of metal deposited in this way from its salts is proportional to the current, has been utilised for measuring the strength of currents with a fine degree of accuracy.  If, for example, the tubes of the voltameter described on page 38 were graduated, the volume of gas evolved would be a measure of the current.  Usually, however, it is the weight of silver or copper deposited from their salts in a certain time which gives the current in amperes.

Electro-plating is the principal application of this chemical process.  In 1805 Brugnatelli took a silver medal and coated it with gold by making it the cathode in a solution of a salt of gold, and using a plate of gold for the anode.  The shops of our jewellers are now bright with teapots, salt cellars, spoons, and other articles of the table made of inferior metals, but beautified and preserved from rust in this way.

Figure 44 illustrates an electro-plating bath in which a number of spoons are being plated.  A portion of the vat V is cut away to show the interior, which contains a solution S of the double cyanide of gold and potassium when gold is to be laid, and the double cyanide of silver and potassium when silver is to be deposited.  The electrodes are hung from metal rods, the anode A being a plate of gold or silver G, as the case may be, and the cathode C the spoons in question.  When the current of the battery or dynamo passes through the bath from the anode to the cathode, gold or silver is deposited on the spoons, and the bath recuperates its strength by consuming the gold or silver plate.

Enormous quantities of copper are now deposited in a similar way, sulphate of copper being the solution and a copper plate the anode.  Large articles of iron, such as the parts of ordnance, are sometimes copper-plated to preserve them from the action of the atmosphere.  Seamless copper pipes for conveying steam, and wires of pure copper for conducting electricity, are also deposited, and it is not unlikely that the kettle of the future will be made by electrolysis.