CHAMBERS'S INFORMATION FOR THE PEOPLE. 



it gives us a uniform standard for comparison of 

 currents. The gases are collected either together 

 in a single vessel, or each in a separate one ; the 

 vessels are graduated, and the number of cubic 

 inches of gas which a current gives in a certain 

 time, say a minute, is an exact measure of its 

 power. 



Any other substances, if in a liquid state, com- 

 posed of two elements, a metal and a non-metal, 

 can be broken up by the current exactly like water. 



The metal always corresponds to the hydrogen, 

 and goes to the negative pole. 



In the electrolysis of a solution of a salt, such as 

 copper sulphate, we have an interesting case. If 

 two platinum poles are put in a solution of this, 

 pure metallic copper is deposited on the negative 

 pole, and free sulphuric acid and oxygen appear 

 at the positive ; and layer after layer of fine 

 copper-dust will be showered on the negative 

 plate, so long as the sulphate of copper lasts. 



In the same way with a solution of nitrate of 

 silver, pure metallic silver will be deposited on the 

 negative pole, and acid and oxygen on the other. 

 All the common metals, gold, silver, platinum, 

 copper, zinc, &c. can be deposited in this manner. 



Electro-metallurgy is the practical application of 

 this power of the galvanic current to precipitate a 

 metallic layer on any surface which forms a nega- 

 tive electrode of a battery or cell. It is divided 

 into two sections electro-typing, where the coating 

 of metal must be firm, and has to be removed from 

 the surface, and electro-plating, when it is to remain 

 fixed on the surface. 



(i.) Electro-typing is the process by which seals, 

 medals, engraved plates, ornaments, &c. can be 

 most accurately copied, and reproduced in metal, 

 more especially copper. We can best explain 

 how this is done by taking a particular instance. 



Suppose we wish to copy a seal in copper. An 

 impression of it is first taken in gutta-percha, 

 sealing-wax, fusible metal, or other substance 

 which takes a sharp impression when heated. A 

 wire to convey the current is stuck into the im- 

 pression while soft, and, if the surface be non- 

 conducting like gutta-percha, it is brushed over 

 with plumbago or black-lead. Then it is attached 

 by its wire to the negative or zinc pole of a weak 

 Daniell's cell, while a plate of copper is attached 

 to the other pole. The two are immersed in a 

 strong solution of copper sulphate. Gradually, the 

 copper from the sulphate is deposited on the 

 black-leaded surface, until in a day or so a toler- 

 ably thick plate is formed. It may easily be 

 detached by the point of a knife, and then we 

 have a faithful copy of the original seal to the 

 minutest line. 



But we may even do without a battery cell 

 altogether, and make a galvanic pair out of the 

 object to be coated and a plate of zinc. 



Electro-typing is of great importance to the 

 engraver. Copper plates may be multiplied to 

 any extent ; and woodcuts, which would soon be- 

 come useless, if printed from directly, can be con- 

 verted into copper plates. Indeed, nearly all the 

 illustrations in popular works are now printed 

 from electro-types. 



(2.) Electro-plating is the art of coating by the 

 galvanic current the baser and cheaper metals, 

 such as brass, bronze, copper, nickel silver, &c. 

 with the more precious ones, such as silver and 

 gold. Theoretically, the process is simple enough, 



27C 



but practically, it requires much experience and 

 skill. When an article is to be electro-silvered, it 

 is first thoroughly cleansed, and then washed with 

 nitrate of mercury. This leaves a thin film of 

 mercury, which acts as a cement between the 

 article and the silver. It is then put in a silver 

 bath, a weak solution of cyanide of silver in 

 cyanide of potassium, and attached to the negative 

 pole of a battery ; while a silver plate is attached 

 to the positive, to keep the solution, by its de- 

 composition, from getting poor. The process ot 

 electro-gilding is very much the same ; only the 

 article first gets a layer of copper by electrolysis, 

 and is then immersed in a gold bath. A piece of 

 gold is suspended from the positive pole, and the 

 bath must in this case be kept hot. 



ELECTRO -MAGNETISM. 



Hitherto, we have considered only the internal 

 powers and properties of the galvanic current ; 

 we now pass to the effects produced by it outside 

 of its path, that is, to its mechanical 'and magnetical 

 effects. The former include the mutual attractions 

 and repulsions of currents, and are usually treated 

 of separately, as electro -dynamics. The latter 

 include all the phenomena where magnetism is 

 produced by the current, and form the subject of 

 electro-magnetism. So closely allied, however, 

 are the two, that the laws of the one class of 

 effects can be deduced from those of the other. 

 Electro-magnetism may thus be employed as in- 

 cluding electro-dynamics. 



The influence of currents on magnets, to which 

 we have already referred in speaking of galvan- 

 ometers, is mutual : magnets act on currents, 

 attracting and repelling them just as currents do 

 on magnets. Not only so, but currents also 

 attract and repel each other according to very 

 simple laws. 



Now, so close is the connection between mag- 

 netic and current action, that there is no reason 

 but to refer them to one and the same ultimate 

 cause. Every case of magnetic action, whether 

 of magnets on currents, currents on magnets, or 

 magnets on magnets, can be at once explained by 

 the theory that magnetism is due to electricity in 

 current. Thus the laws of current action are at 

 the foundation of electro-magnetism. They are 

 two, though one is practically included in the 

 other : (i.) Two parallel wires, capable of motion, 

 or two flexible conducting chains, attract each 

 other when currents pass along them in the same 

 direction, and repel when they pass in opposite 

 directions. (2.) Two wires or chains, not parallel, 

 but crossing each other, attract when the current 

 runs in both, either to or from the point of crossing, 

 but repel when it flows in one to, and in the other 

 from, this point. 



These laws are easily proved by experiment 

 with light frames of copper wire. They must be 

 nicely movable upon pivots, and at the same time 

 allow the current to pass round the frames without 

 interruption. Many mechanical actions can be 

 produced by the application of the two laws we 

 have given. For example, a current flowing in a 

 circle, may be made to cause a constant rotation 

 of a wire conveying a current at right angles to 

 the former. 



A similar rotation may be produced by putting 

 for the circular current, in the last case, a pole of 



