Electroplating on a commercial scale is of course a much more elaborate process, but the principle remains exactly the same. Fig. 36 shows the general arrangement of a plating shop. It is obviously extremely important that the deposit on a plated article should be durable, and to ensure that the coating will adhere firmly the article must be cleaned thoroughly before being plated. Cleanliness in the ordinary domestic sense is not sufficient, for the article must be chemically clean. Some idea of the care required in this respect may be gained from the fact that if the cleaned surface is touched with the hand before being plated, the coating will strip off the parts that have been touched. The surfaces are first cleaned mechanically, and then chemically by immersion in solutions of acids or alkalies, the cleaning process varying to some extent with different metals. There is also a very interesting process of cleaning by electricity. The article is placed in a vat fitted with anode and cathode rods, just as in an ordinary plating vat, and containing a solution of hydrate of potash and cyanide of potassium. The anode consists of a carbon plate, and the article is hung from the cathode rod. Sufficient current is passed through the solution to cause gas to be given off rapidly at the cathode, and as this gas rises to the surface it carries with it the grease and dirt from the article, in the form of a dirty scum. After a short time the article becomes oxidized and discoloured, and the current is then reversed, so that the article becomes the anode, and the carbon plate the cathode. The current now removes the oxide from the surface of the article, which is left quite bright and chemically clean. [Illustration: _By Permission of_] [_W. Canning & Co._ FIG. 36.--General Arrangement of an Electroplating Shop.] When thoroughly cleaned the articles are ready to be placed in the plating vats. These vats are usually made of wood lined with chemically pure lead, or of iron lined with enamel or cement. Anode and cathode rods made of brass are placed across the vats, and from these the anodes of the various metals and the articles to be plated are hung by hooks of nickel or brass. Any number of rods may be used, according to the size of the vat, so long as the articles have an anode on each side. If three rods are used the articles are hung from the centre one, and the anodes from the outside ones. If a number of small articles are to be plated together they are often suspended in perforated metal trays. Small articles are also plated by placing them in a perforated barrel of wood, or wood and celluloid, which revolves in the solution. While the articles are being plated the revolving of the barrel makes them rub one against the other, so that they are brightly burnished. Dog chains, cycle chain links, button-hooks, and harness fittings are amongst the articles plated by means of the revolving barrel.