PRINTING 



215 



Practically all silver, gold, and nickel plating is done in this 

 way; machine, bicycle, and motor attachments are not solid 

 nickel, but are of cheaper material electrically nickelplated. 

 When spoons are to be plated, they are hung in a solution of 

 silver salts side by side with a thick slab of pure silver, as in 

 Figure 83 . The spoons are connected with the negative terminal 

 of the battery, while the slab of pure silver is connected with the 

 positive terminal. The length of 

 time that the current flows deter- 

 mines the thickness of the plating. 



How pure metal is obtained from 

 ore. When ore is mined, it con- 

 tains in addition to the desired 

 metal many other substances. In 

 order to separate out the desired 

 metal, the ore is placed in some suit- 

 able acid bath, and is connected with 

 the positive terminal of a battery, 

 thus taking the place of the silver 

 slab in the last section. When cur- 

 rent flows, any pure metal which is 

 present is dissolved out of the ore 



und is deposited on a convenient negative electrode, while the 

 impurities remain in the ore or drop as sediment to the bottom 

 of the vessel. Metals separated from the ore by electricity are 

 called electrolytic metals and are the purest obtainable. 



Printing. The ability of the electric current to decompose 

 a liquid and to deposit a metal constituent has practically 

 revolutionized the process of printing. Formerly, type was 

 arranged and retained in position until the required number of 

 impressions had been made, the type meanwhile being unavail- 

 able for other uses. Moreover, the printing of a second edition 

 necessitated practically as great labor as did the printing of the 



FIG. 83. Plating spoons by 

 electricity. 



