[Illustration:  FIG. 209.—­Plating spoons by electricity.]

Practically all silver, gold, and nickel plating is done in this way; machine, bicycle, and motor attachments are not solid, but are of cheaper material electrically plated with nickel.  When spoons are to be plated, they are hung in a bath of silver nitrate side by side with a thick slab of pure silver, as in Figure 209.  The spoons are connected with the negative terminal of the battery, while the slab of pure silver is connected with the positive terminal of the same battery.  The length of time that the current flows determines the thickness of the plating.

294.  How Pure Metal is obtained from Ore.  When ore is mined, it contains in addition to the desired metal many other substances.  In order to separate out the desired metal, the ore is placed in some suitable 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 current flows, any pure metal which is present is dissolved out of the ore and 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.

295.  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 unavailable for other uses.  Moreover, the printing of a second edition necessitated practically as great labor as did the first edition, the type being necessarily set afresh.  Now, however, the type is set up and a mold of it is taken in wax.  This mold is coated with graphite to make it a conductor and is then suspended in a bath of copper sulphate, side by side with a slab of pure copper.  Current is sent through the solution as described in Section 293, until a thin coating of copper has been deposited on the mold.  The mold is then taken from the bath, and the wax is replaced by some metal which gives strength and support to the thin copper plate.  From this copper plate, which is an exact reproduction of the original type, many thousand copies can be printed.  The plate can be preserved and used from time to time for later editions, and the original type can be put back into the cases and used again.

CHAPTER XXXII

MODERN ELECTRICAL INVENTIONS

296.  An Electric Current acts like a Magnet.  In order to understand the action of the electric bell, we must consider a third effect which an electric current can cause.  Connect some cells as shown in Figure 200 and close the circuit through a stout heavy copper wire, dipping a portion of the wire into fine iron filings.  A thick cluster of filings will adhere to the wire (Fig. 210), and will continue to cling to it so long as the current flows.  If the current is broken, the filings fall from the wire, and only so long as the current flows through the wire does the wire have power to attract iron filings.  An electric current makes a wire equivalent to a magnet, giving it the power to attract iron filings.