Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/127

metals, such as those of silver. Sometimes only part of an object is to be gilt, such as the inside of a silver- plated cream-jug ; in this case the vessel would be filled with the gilding solution, in which the anode of the battery is immersed. Gold is sometimes deposited not as a coating upon other metals, but as an electrotype in gutta-percha or in plaster moulds; small objects of elaborate workmanship being thus produced in solid gold, without the workman-

ship of the chaser and engraver.

Although copper, silver, and gold are the metals to which the attention of the electro-metallurgist is usually restricted. it should be remembered that he is also able to obtain clectro-deposits of a very large number of other metals. .\[any of these are not practically used, but one of them has of late years become of considerable importance. This is the metal m'cl'el In 1869 Dr Isaac Adams of Boston, l'nited States, patented a process for depositing nickel from solutions of various double salts; but Dr Gore had many years previously employed similar salts in England, and had published the results of his experiments. The deposi- tion of nickel, eSpecially from the sulphate of nickel and ammonium, is now carried out on a large scale both in England and in the United States. The metal is deposited as a very thin but excessively hard coating, and has the advantage of not readily tarnishing or eorroding even in a moist atmosphere. Hence it has become common to eieetro~ nickel iron and steel objects for use on board ship, as well as gun-barrels, sword-scabbards, harness furniture, gas- burners, and various articles for household use.

Iron, like nickel, may be deposited from its double salts, and excellent results have been obtained by Klein, of St Petersburg, with the double sulphate of iron and ammonium. Engraved copper-plates are much harder when faced with electro-deposited iron than when unprotected, and they Consequently yield a much larger number of im- pressions before losing their sharpness. Plates for printing bank-notes have been treated in this way.

Not only can the electro-metallurgist deposit simple metals, such as those noticed above, but he is able likewise to deposit certain alloys, such as brass, bronze, and German silver. The processes by which this can be effected are not, however, very generally used.

Among the minor applications of electro-metallurgy we may mention the process of electrotyping flowers, insects, and other delicate natural objects. These are first dipped for a moment in a warm solution of nitrate of silver in alcohol, and then exposed to a reducing liquid, such as a solution of phosphorus in bisulphide of carbon ; an electro- deposit may then be thrown down upon this metallized surface. Daguerreotypes are sometimes improved by coat- ing them with a very delicate film of electro-deposited gold. Again, in some of the modern photographic processes for printing, copper electrotypes are taken directly or indirectly from the bichromatized gelatiiie. Of late years, too, a method of refining crude copper by means of electro- metallurgy has been introduced, and is now successfully carried out on a large scale. Slabs of blister-copper are plunged into a solution of sulphate of copper, and form the anodes of a battery; the copper then dissolves, and is de- posited iii a condition of great purity at the opposite pole, most of the impurities sinking to the bottom of the depositing vat. The process should be restricted to copper which is free from any metals likely to be deposited along with the metal under purification.

It has been considered desirable not to include within the limits of this article any of the numerous formulae for preparing the solu- tions used by electro-metallurgists. For these, and for other details, see the treatises of G. Core (1877), J. Napier (5th ed.. 1876), A- Watt (5th ed., 1874), A. Smec (3rd ed., 1851), and G. Shaw (1844); U._ V. “'alker's Electrotype Manipulation (1850); and II. Dirck‘s History of Electra-metallurgy (1863).

(f. w. r.*)

ELECTROMETER. An electrometcr, according to Sir Wm. Thomson, who is the greatest living authority on this subject, and has done more than any one else to perfect this kind of physical apparatus, is “an instrument for measuring differences of electric potential between two con- ductors through the effects of electrostatic force.” A gal- vanonieter, on the other hand, which might also be defined as an instrument for measuring differences of electric poten- tial, idilizes the electromagnetic forces due to the currents produced by differences of electric potential. An instru- ment designed merely to imlicalc, without measuring, differ- ences of electric potential is called an elech'oscope It is obvious that every electrometer may be used as an electro- scope, and it is also true that all electroscopes are electro- mcters more or less ; but the name electrometer is reserved for such instruments as have a scale enabling us, either directly or by appropriate reduction, to refer differences of potential to some unit.

The modern electrician is far more concerned with measurements of electric potential than with measurements of electric quantity; and consequently all modern electrometric instruments are suited for direct measure- ments of the former kind. It is only indirectly that such instruments measure electric quantity. With the older electricians it was otherwise; and some of the earliest electrometcrs were designed for the direct measurement of quantity.

Such was the measuring jar of Lane,^[1] represented in fig. 1 (after Lane’s D is a Leydcn jar, fastened to a stand in such a way that jar. iiess). its outer armature can be insulated or connected to earth at will. The inner armature is in good metallic connection with the knob C. A horizontal metal piece A is mounted on a glass pillar, and carries another knob, which can be set at any required distance from C by means of a screw and gra- duation. The piece A is connected with the outer armature of the jar by a thin wire B contained in a glass tube. This last piece was added by Riess,^[2] whose arrangement of the apparatus we have been describing. One way of using the in- strument is as follows. The halls are set at a convenient distance apart, the stand is carefully insulated, and the outer armature of the jar connected with the battery of jars or other system to be . I _ charged, and the inner armature with the source of electricity, say the prime conductor of an electric machine. The electriCity accu- mulates on the inner armature till a certain difference of potential between C and A is reached, and then a certain quantity 4] of_elec tricity passes from C to A in the form of a spark, after which a quantity q remains distributed between the outer armature and the accumulator which is being charged. This process is continued, and as each spark passes, a quantity 4] is added to the charge on the outer armature and accumulator. Hence if the capacity of the outer armature be negligible compared with that of the accumu- lator, the charge of the latter will be proportional to the number of sparks between the balls. The measuring jar may also be used to measure the overflow of electricity from one armature of. an accumulator when the other is connected with an electric machine. In this case the outer coating of the jar is connected “'1”! the earth. and C is connected with the armature of the accumulator. There is no occasion to discuss minutely here the corrections necessarv ln'tlle latter method of using the apparatus ; on these and kindred points

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1. Phil. Trans., 1769.
2. The object of the fine wire is to absorb the energy of the_discharges, and prevent the disintegration of the metal of the balls which renders the action of the apparatus irregular (sec Ricss, Reibungselectncitdt, § 386).