The Boy's Playbook of Science Part 25

_Tenth Experiment._

[Ill.u.s.tration: Fig. 188. A. A single cell, Daniell's, attached to B, the trough containing the mould and the plate of copper. Below is a Smee's battery ready to be attached to a larger trough for the purpose of electrotyping a great number of moulds at the same time.]

A single cell apparatus is only adapted to produce small electrotypes, but when larger ones are required, a separate battery of three or four [Page 202] Daniell's or Smee's cells is required; and it is usual to place the mould to be copied in a separate wooden trough, attaching it to the cathode wire, whilst a copper plate is connected with the anode, so that as the solution of sulphate of copper undergoes decomposition by the pa.s.sage of the electricity, it is kept almost in a normal state, in consequence of the oxygen of the water and the acid pa.s.sing to the copper plate, which they attack and dissolve as fast as the oxide of copper and hydrogen are liberated at the cathode, where the latter deoxidizes the oxide of copper, and by a secondary action deposits metallic copper; the object being to dissolve fresh metal as the copper is deposited on the mould. (Fig. 188.)

_Eleventh Experiment._

To silver electrotypes or other bra.s.s and copper articles, the first attention must be paid to the cleanness of them; and when an electrotype is just removed from the copper solution, and washed in clean water, it is at once ready to receive the coating of silver; otherwise, if it has been handled, or is slightly greasy, it should be first boiled in a solution of common was.h.i.+ng soda, and then the oxide removed by pa.s.sing it rapidly in and out of some "Dipping Acid," which is prepared by mixing together equal parts of oil of vitriol and nitric acid; when removed from the "Dipping Acid," it must be well washed in water, and may remain under the surface of the water until the silvering solution is ready. A silver solution may be prepared by dissolving a sixpence in some nitric acid contained in a flask; it is then poured into a solution of common salt, which precipitates the chloride of silver, and leaves the copper in solution--the latter is poured off when the chloride has subsided, and after being well washed in some boiling water, is dissolved in a solution of cyanide of pota.s.sium. If a clean electrotype is plunged into this solution, it is immediately covered with a very thin coating of silver, which of course would soon wear off, and in order to increase the thickness of the silver deposit, a single cell arrangement may be constructed of a large gallipot containing a wide porous cell and a circle of amalgamated zinc around it; the arrangement is set in action by pouring a solution of salt (or, still better, sal ammoniac) into and around the porous vessel, and the silvering solution into the latter; a connecting wire pa.s.ses from the zinc, and the article being attached to it, is now plunged into the porous cell, when a current of electricity slowly pa.s.ses and deposits the silver on the copper article. (Fig. 189.)

[Ill.u.s.tration: Fig. 189. The gallipot containing the solution of sal ammoniac, with the circular amalgamated zinc with wire and binding screw to which the medal is attached, and contained in the porous vessel holding the silvering solution and medal.]

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_Twelfth Experiment._

Separate batteries and large troughs containing a solution of cyanide of silver in cyanide of pota.s.sium are used on a grand scale in the electro-plating establishment of Messrs. Elkington of Birmingham, where the finest specimens of the art are to be obtained; a plate of silver being attached to the anode to supply the loss of silver in these troughs.

_Thirteenth Experiment._

The art of gilding by the agency of electricity is quite as simple as the processes already described, although greater care is necessary to avoid any loss of the precious metal. A small bit of gold is dissolved in a mixture of three parts muriatic acid and one of nitric acid, which forms the chloride of gold. This is then digested with an excess of calcined magnesia, and the gold is precipitated as an oxide of the metal; the latter is collected and washed, and then boiled in strong nitric acid to remove the magnesia clinging to it, and being again thoroughly washed with water, is dissolved in a solution of cyanide of pota.s.sium, forming a solution of cyanide of gold and pota.s.sium, which may be placed in the porous cell of the single cell arrangement already described in the Eleventh Experiment.

_Fourteenth Experiment._

The safest and surest mode of making a gilding solution is to dissolve some cyanide of pota.s.sium in water in a gallipot, and having placed a porous vessel therein containing the same solution, put a plate of copper into the porous cell, and some thin foil of pure gold into the gallipot; connect the gold with the anode of a single cell of Daniell, and the copper in the porous cell with the cathode, and in a few hours sufficient gold will be dissolved for the purpose of gilding.

It is usually recommended to warm the gilding solution till it reaches a temperature of about 150 Fahr., and a very moderate battery power is employed in Electro Gilding. Indeed the same arrangement, shown in the Eleventh Experiment, Fig. 189, will also answer for the gilding solution. After being gilt, the articles may be rubbed with a little tripoli, or burnished (with taste) by the handle of a key.

_Fifteenth Experiment._

Pa.s.sing on to the more brilliant results obtainable from a powerful voltaic battery (of at least thirty pairs of Grove), the beautiful incandescence of platinum wire may first be noticed. If a wire of this metal is stretched between the bra.s.s standards of two ring stands, the length must be proportioned to the power of the battery; the adjustment can be made very conveniently by twisting the platinum wire on one ring stand, and then leaving the other end loose, the second ring stand may be brought nearer and nearer to the first, until the desired intensity of [Page 204] light from the incandescent wire is obtained. (Fig. 190.) If the wire is contained in a gla.s.s tube the cooling effect of currents of air is prevented, and a much greater length of wire can be made hot.

[Ill.u.s.tration: Fig. 190. A A. Two ring stands with the battery wires B B (which should be a convenient length) attached. C. Platinum wire, fixed end. D. The other end held in one hand and shortened as the stand is moved by the other hand.]

_Sixteenth Experiment._

With the same arrangement, a chain composed of alternate links of silver and platinum wire presents a very pretty effect, every alternate link of platinum being incandescent, whilst the silver, from its excellent conducting power, remains comparatively cool.

_Seventeenth Experiment._

Fireworks or gunpowder, arranged in proper cases, are fired at a great distance from the voltaic battery by heating a thin iron or platinum wire contained within them by the pa.s.sage of the electricity; and submarine and other explosions of gunpowder by the same agency have become a common engineering operation. (Fig. 191.)

[Ill.u.s.tration: Fig. 191. A. A Gerb firework with two holes punctured, through which the bit of iron wire pa.s.ses, and is wound round the battery wires tied to the outside of the case. C. A gut bladder containing the thin wire and powder for a miniature submarine explosion.]

During the operation of blasting the hard marl rocks in the River Severn by Mr. Edwards, C.E., a number of holes were made side by side in the bed of the river, and cartridges formed of strong duck or canvas, tapered at the bottom, were filled with charges of powder from two to four pounds, according to the depth of the marl; thus, two pounds for four feet, three pounds for four feet six inches, and four pounds for five feet. Into the bag were conveyed the wires of the voltaic battery, or Bickford's fuse, and being then coated with pitch and tallow, and finally greased all over and dusted with whitening, they rarely failed, and were all fired simultaneously under water. The pitch and tallow first, and afterwards the simple tallow, effectually excluded the water from the gunpowder contained in the canvas bag.

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_Eighteenth Experiment._

The burning of various metals by the battery is displayed with great effect by De la Rue's discharger, as also the incandescence of the charcoal points producing the _electric light_. The illuminating power derived from a forty-cell Grove's battery of the ordinary size is about equal to the light of 500 candles.

[Ill.u.s.tration: Fig. 192. De la Rue discharger, containing a series of six pairs of different substances, such as charcoal, iron, lead, zinc, copper, antimony, in six pair of crayon holders, and turning on a centre, so as to be changed at pleasure.]

Fizeau and Foucault have made a careful comparison of the light obtained from 92 carbon couples as arranged in a Bunsen's battery, and of the oxy-hydrogen, or Drummond Light, as compared with that of the sun, and they state that "On a clear August day, with the sun two hours high, the electric light (a.s.suming the sun as unity) bore to it the ratio of one to two and a half--_i.e._, the sun was two and a half times more powerful, while the Drummond Light was only 1/146th that of the sun."

Bunsen found the light from 48 carbons equal to 572 candles. In Bunsen's battery carbon is subst.i.tuted for the platinum in Grove's arrangement; and simultaneously with Bunsen, Cooper (in England) had applied charcoal for the same purpose.

At night the giant s.h.i.+p (Polyphemus like) is to have an electric light at the mast-head whilst steaming across the Atlantic.

[Ill.u.s.tration: Fig. 193. _Great Eastern_, with electric light.]

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CHAPTER XV.

MAGNETISM AND ELECTRO-MAGNETISM.

If a small helix, or coil of covered wire, is arranged with an unmagnetized steel needle within it, so that the discharge of a large Leyden jar may take place through the coil, the needle will be found strongly magnetic after the discharge of the electricity. (Fig. 194.) Many years before this was known, it had been noticed that when a s.h.i.+p was struck by lightning, the compa.s.ses were generally reversed; and in a special case, where a house was struck, the electricity entered a box of knives, fusing some, tearing the handles off others, but leaving them strongly magnetic. Electricians tried to repeat the effect by sending the discharge of powerful Leyden batteries through bars of steel without any important result; and it was not until Oersted, in the year 1819, made his important discovery that the copper wire conveying the electricity possessed peculiar magnetic power, that the principle began to be understood, and then the electricians succeeded in imitating the effects of lightning on steel, as already described in the beginning of this chapter. (Fig. 194.)

[Ill.u.s.tration: Fig. 194. A A. A gla.s.s tube supported on two uprights of wood, with coil of copper wire pa.s.sing round it, terminating in the b.a.l.l.s B B. C. Needle to place inside gla.s.s tube.]

[Ill.u.s.tration: Fig. 195.]

When the electricity has pa.s.sed away from the Leyden jar through the coil of [Page 207] copper wire, it no longer possesses any power to affect a piece of steel or iron, but if the wires of the voltaic battery are now connected with the coil of copper wire, which should be covered with cotton or silk, and many yards in length, then a bar of steel or soft iron is not only rendered magnetic, but remains permanently so, as long as the current of electricity continues to pa.s.s along the coil of wire, so that if some nails or iron filings are brought to the bar of iron, one end of which projects from the coil, they cling to it with great force, and a great number of nails may be hung on in this manner, but they immediately fall off when the contact is broken with the battery. (Fig. 195.)

Electricity thus becomes a source of magnetism, and the discoverer, Oersted, found that only needles or bars of steel or iron were thus affected, and not those of bra.s.s, sh.e.l.l-lac, sulphur, and other substances; he termed the conducting wire "a conjunctive wire," and described the effect of the electric current or "_electric conflict_,"

as he called it, as resembling a Helix (from [Greek: _helisso_], to turn round; a screw or spiral), and that it is not confined to the conducting wire, but radiates an influence at some distance. This latter statement is exactly in accordance with our present notions, and hence the coil conveying the current is said to _induce_ magnetism in the iron or steel, just as the phenomena of induction are produced with frictional electricity. The effect of Oersted's discovery, says Silliman, was truly _electric_; the scientific world was ripe for it, and the truth he thus struck out was instantly seized upon by Arago, Ampere, Davy, Faraday, and a crowd of philosophers in all countries. The activity with which this new field of research has been cultivated, has never relaxed even to this hour, while it has borne fruit in a mult.i.tude of theoretical and practical truths, and above all, in the electro-magnetic telegraph, truly called, and especially in connexion with the Atlantic telegraph wire, "_the great international nerve of sensation_."

[Ill.u.s.tration: Fig. 196. A loadstone mounted in bra.s.s or silver, with the iron cheeks B B attached. C. The bit of soft iron called the armature.]

Magnetism is not only the result of a current of electricity through any good conductor, but there are certain oxides of iron, called magnetic iron ores, which have the property of attracting iron filings, and are mostly found in primitive rocks, being abundant at Roslagen, in Sweden, and called the loadstone, from its always pointing, when freely suspended, to the Polar, North, or Load Star. If a tolerably large specimen of this mineral is examined, there will be found usually two points where the iron filings are attracted in larger quant.i.ties than in other parts of the same specimen. These attractive points are called poles, and the loadstone being properly mounted with soft iron bars, termed cheeks, bound round it (in old-fas.h.i.+oned loadstones) with silver plate and duly ornamented with [Page 208] engraving, has its magnetic power greatly increased, and is then said to be endowed with magnetic polarity; and to prevent the loss of power, a soft piece of iron, called the armature, is placed across and attracted to the poles of the loadstone. (Fig. 196.)

_Second Experiment._

If a needle of tempered steel (fitted with a little bra.s.s cup in the centre to work upon a point) is rubbed with the loadstone in one direction only, it is rendered permanently magnetic, and will now be found to take a certain fixed position, pointing always in a direction due north and south. The end which points towards the north is called the north pole, and the other extremity the south pole, and it is usual to mark the north pole with an indent or scratch to distinguish it at all times.