through the cell, the mould being the kathode; a plate of copper being employed as an anode, dissolving gradually into the liquid at a rate exactly equal to the rate of deposition at the kathode. This use of a separate battery is more convenient than producing the electrotypes in the actual cell of a Daniell's battery. The process is largely employed at the present day to reproduce repoussé and chased ornament and other works of art in facsimile, and to multiply copies of wood blocks for printing. Almost all the illustrations in this book, for example, are printed from electrotype copies, and not from the original wood blocks, which would not wear so well.

421. Electroplating.-In 1801 Wollaston observed that a piece of silver, connected with a more positive metal, became coated with copper when put into a solution of copper. In 1805 Brugnatelli gilded two silver medals by making them the kathodes of a cell containing a solution of gold. Messrs. Elkington, about the year 1840, introduced the commercial processes of electroplating. In these processes a baser metal, such as German silver (an alloy of zinc, copper, and nickel) is covered with a thin film of silver or gold, the solutions employed being, for electro-gilding, the double cyanide of gold and potassium, and for electro-silvering the double cyanide of silver and potassium.

Fig. 159 shows a battery and a plating-vat containing the silver solution. From the anode is hung a plate of metallic silver which dissolves into the liquid. To the kathode are suspended the spoons, forks, or other articles which are to receive a coating of silver. The addition of a minute trace of bisulphide of carbon to the solution causes the deposited metal to have a bright surface. If the current is too strong, and the deposition too rapid, the deposited metal is grayish and crystalline.

In silvering or gilding objects of iron it is usual first to plate them with a thin coating of copper. In gilding

base metals, such as pewter, they are usually first copper-coated. The gilding of the insides of jugs and cups is effected by filling the jug or cup with the gilding solution, and suspending in it an anode of gold, the vessel itself being connected to the - pole of the battery.

Instead of a battery a thermo-electric generator (Art. 384), or a dynamo-electric generator (Art. 408), is now frequently employed.

422. Metallo-chromy.-In 1826 Nobili discovered that when a solution of lead is electrolysed a film of peroxide of lead forms upon the anode. If this be a sheet of metal, a plate of polished steel, for instance,—placed horizontally in the liquid beneath a platinum wire as a kathode, the deposit takes place in symmetrical rings of varying thickness, the thickest deposit being at the centre. These rings, known as Nobili's rings, exhibit all the tints of the rainbow, owing to interference of the waves of light occurring in the film causing rays of different wave-length and colour to be suppressed at different distances from the centre. The colours form, in fact, in reversed order, the "colours of thin plates" of Newton's rings. According to Wagner this production of chromatic effects by electrolysing a solution of lead in caustic soda, is applied in Nuremberg to ornament metallic toys. The author of these Lessons has observed that when Nobili's rings are made in a magnetic

field they are no longer circular, the depositing currents being drawn aside in a manner which could be predicted from the observed action of magnets on conductors carrying currents.

422 (bis). Electro - Chemical Power of Metals.-The following Table gives the electromotive-force of the different metals as calculated by the method of Art. 414 from their electro - chemical equivalents (Art. 212), and from the heat evolved by the combination with oxygen of a portion of the metal equivalent electro-chemically in amount to one gramme of hydrogen. The electromotive-forces (in volts) as observed (in dilute sulphuric acid) are added for comparison.

The order in which these metals are arranged is in fact nothing else than the order of oxidisability of the metals (in the presence of dilute sulphuric acid); for that metal tends most to oxidise which can, by oxidising, give out the most energy. It also shows the order in which the metals stand in their power to replace one another (in a solution containing sulphuric acid.) In this order too, the lowest on the list first, are the metals deposited by an electric current from solutions containing two or more of them: for that metal comes down first which requires the least expenditure of energy to separate it from the elements with which it was combined.

CHAPTER XII.

TELEGRAPHS AND TELEPHONES.

LESSON XXXIX.-Electric Telegraphs.

423. The Electric Telegraph.-It is difficult to assign the invention of the Telegraph to any particular inventor. Lesage (Geneva, 1774), Lomond (Paris, 1787), and Sir F. Ronalds (London, 1816), invented systems for transmitting signals through wires by observing at one end the divergence of a pair of pith-balls when a charge of electricity was sent into the other end. Cavallo (London, 1795) transmitted sparks from Leyden jars through wires "according to a settled plan." Soemmering (Munich, 1808) established a telegraph in which the signals were made by the decomposition of water in voltameters; and the transmission of signals by the chemical decomposition of substances was attempted by Coxe, R. Smith, Bain, and others. Ampère (Paris, 1821) suggested that a galvanometer placed at a distant point of a circuit might serve for the transmission of signals. Schilling and Weber (Göttingen, 1833) employed the deflections of a galvanometer needle moving to right or left to signal an alphabetic code of letters upon a single circuit. Cooke and Wheatstone (London, 1837) brought into practical application the first form of their needle telegraph. Henry (New York, 1831) utilised the attraction of an electromagnet to transmit signals, the movement of the armature producing audible sounds according to a certain code. Morse (New York, 1837) devised a telegraph in which the attraction of an armature by an electromagnet was made to mark a dot or a dash upon a moving strip of paper. Steinheil (Munich, 1837) discovered that instead of a return-wire the earth might be used, contact being made to earth at the two ends by means of earth

plates (see Fig. 160) sunk in the ground. Gintl (1853) and Stearns (New York, 1870) devised methods of duplex signalling. Stark (Vienna) and Bosscha (Leyden, 1855) invented diplex signalling, and Edison (Newark, N. J., 1874) invented quadruplex telegraphy. For fast-speed work Wheatstone devised his automatic transmitter, in which the signs which represent the letters are first punched by machinery on strips of paper; these are then run at a great speed through the transmitting instrument, which telegraphs them off at a much greater rate than if the separate signals were telegraphed by hand. Hughes devised a type-printing telegraph. Wheatstone invented an ABC telegraph in which signals are spelled by a hand which moves over a dial. For cable-working Sir W. Thomson invented his mirror galvanometer and his delicate siphon-recorder. It is impossible in these Lessons to describe more than one or two of the simpler and more frequent forms of telegraphic instruments. Students desiring further information should consult the excellent manuals on Telegraphy by Messrs. Preece and Sivewright, and by Mr. Culley.

424. Single-Needle Instrument. The singleneedle instrument (Fig. 160) consists essentially of a

vertical galvan

ometer, in which a lightly hung magnetic needle is deflected to right or left when a current is sent, in one direction or the other, around a coil surrounding the needle; the needle visible in front of the dial is but an index,