817 



ELECTRO-MAGNETISM. 



ELECTRO-METALLURGY. 



813 



It was afterwards found that the magnetic needle of the multiplier 

 could be acted on by electrical discharges from a Leyden jar ; and Mr. 

 Faraday showed conclusively that, with "the condition of time, ordi- 

 nary electricity can produce a continued deviation of the needle ; this 

 condition he fulfilled by making the electricity pass through imperfect 

 conductors. 



Arago observed that small fragments of soft iron were attracted by 

 the conductor of the galvanic pile, and the same current imparted 

 permanent magnetism to small needles of steel. The needle should 

 be placed perpendicularly to the joining wire or current, or, which is 

 better, be introduced in a helix, the discharge of the current through 

 which instantaneously magnetises the needle. 



Nobili observed that needles placed between the isolated spires of a 

 plane spiral of copper wire were, by an electrical discharge, magnetised 

 in opposite ways, when near the centre and when near the circum- 

 ference. Savary also observed that when needles were placed hori- 

 zontally with their middle points vertical over a horizontal current and 

 the needles perpendicular to the direction of the current, they were 

 differently magnetised according to their distances. These experi- 

 ments he has varied relatively to the length of the needles, the length 

 and diameter of the conductor, &c. 



The magnetising force of the current is transmitted without sensible 

 loss through isolating media, as glass, wood, &c., but is much altered 

 by the interposition of conducting plates, a result similar to the de- 

 velopment of ordinary electricity by the influence of electrised bodies. 

 Thus a large plate interposed between the conductor and the needles 

 weakens the magnetising effect of feeble discharges, while it augments 

 strong ones : and for a given charge, a thin and a broad interposed 

 conducting plate may produce contrary effects, and with a certain 

 determinate breadth the effect would be unaltered, and in general the 

 two surfaces of the same plate exercise contrary actions. (Savary.) 



When a bar of soft iron, bent in a horse-shoe shape, is encompassed 

 by a helix covered with silk and always turned in the same way, it 

 may be made to receive a powerful magnetism under the influence of 

 a current through the helix discharged from a voltaic battery. In 

 this way electro-magnets have been formed capable of supporting a 

 ton weight and upwards. The magnetism of the soft iron under the 

 influence of the voltaic current, attains its maximum in a few momenta, 

 and ceasea immediately when contact between the helix and the 

 battery ia broken. On reversing the direction of the current through 

 the helix the polarity of the electro-magnet is also reversed. It is 

 remarkable that the molecular action which we know must take place 

 in magnetising a bar of soft iron has the effect of increasing its length, 

 and at the game time causing it to emit an audible sonnd, the note 

 being such as would be produced by striking one end of the bar. The 

 name note ia also produced on breaking contact with the battery when 

 the bar returns to its original dimensions. If these molecular move- 

 ment* be quickly alternated by making and breaking contact in quick 

 succession, there U an elevation of temperature in the bar which is 

 quite independent of the heat of the conducting wire caused by the 

 current. 



The great use which ia made of the electro-magnet in telegraphy 

 [TELEGBAFH] has led to considerable research as to the most favour- 

 able conditions for forming it. According to Lenz and Jacob!, when 

 the voltaic current is of uniform strength the magnetism induced is 

 directly proportional to the number of coils on the bar, which coils 

 may be distributed over its whole length or be accumulated towards its 

 two extremities. Nor is the effect influenced by the diameter of the 

 surrounding coils, for although the induction diminishes as the distance 

 of the magnet from the wire, yet the induction produced by the 

 increased length of the wire in the circumference of the coil is aug- 

 mented in the same proportion. The thickness of the wire is a matter 

 of indifference, the energy of the magnetism being cceterit paribus pro- 

 portional to the strength of the current, it being directly as the electro- 

 motive force, and inversely as the resistance of the circuit. The 

 retentive power of the magnet, like the attractive power of electricity, 

 increases as the square of the intensity of the magnetism. The magnetic 

 effect of a solid bar is proportional to its surface ; in cylindrical bars it 

 U u the square of the weight. Now as bundles of insulated wires 

 expose a larger amount of surface than a solid bar, they are capable of a 

 higher magnetic result than a solid bar of equal weight. It is also stated 

 that the only advantage of long bars over short ones is the removal 

 to a greater distance of the two opposite poles from each other. 



The discovery of the currents produced by volta-electric induction 

 is due to Mr. Faraday. With about 203 feet of copper wire he formed 

 each of two helices, and twisted them about a cylinder of wood, making 

 one in communication with a galvanometer and the other with a 

 powerful voltaic pile. The moment the communication was esta- 

 blished, the galvanometer deviated; then, after some oscillations, 

 returned to its place, and again deviated the instant this communica- 

 tion wan broken : hence the directions of the inducing and induced 

 current* are contrary, while that generated at the interruption of com- 

 munication or ceiaation of the inducing current if directed the same 

 way with the latter. 



The same philosopher has also succeeded in producing currents by 

 the influence of magnets, his experiments with the great magnets of 

 the Royal Society proving most manifestly the disengagement of elec- 

 tricity by the influence of ordinary magnetism. The extraction of the 



ARTS ASD SCI. DIV. VOL. in. 



electrical spark from the magnet, as also the continued rotations pro- 

 duced by terrestrial magnetism, are well-known phenomena. [MAG- 

 NETO-ELECTRICITY.] The theory of Ampere, which supposes electrical 

 currents to exist round the component particles of magnetised sub- 

 stances, and round the mass of the earth, is perhaps the most satis- 

 factory explanation yet given of the cause of magnetic action, and has 

 been greatly strengthened by the discoveries of Faraday on electro- 

 magnetic induction, by which many objections that had been urged 

 against this theory are removed. 



The varied phenomena of electro-magnetism require a good deal of 

 delicately constructed apparatus, a knowledge of the subject, and much 

 careful manipulation, for an account of which we may refer to almost 

 any competent work on natural philosophy. 



ELECTRO-METALLURGY. The chief object of this branch of 

 applied science is the deposition of a thin metallic layer on the surface 

 of some other body prepared for its reception ; the outer layer being 

 intended generally (but not always) for one of three purposes tho 

 decoration, the protection, or the imitation of that which lies beneath. 

 The mode of proceeding assumes different forms, according to the 

 purpose in view. The names Electrotype, Galranotype, Voltatype, Vol- 

 tayraphy, Galrano-Plastics, Electro-Plating and Gilding, have been all 

 applied in a somewhat confused manner to different modifications of 

 the art ; but we agree with Mr. Smee, that it would be convenient 

 to adopt the term Electro-Metallurgy as a general one, including all or 

 most of the varieties. 



Under the article GALVANISM will be found an account of the prin- 

 ciple of what are termed batteries, or arrangements for generating and 

 sustaining electro-chemical currents. If the wire connected with the 

 zinc element of a compound battery have a plate or surface of any 

 conducting substance attached to it, moulded into any form, or having 

 any design or pattern cut upon it, while the other wire terminates in a 

 copper plate of equal surface with the former, both being placed near 

 and parallel to each other, but not in contact, and bo immersed in a 

 saturated solution of sulphate of copper, this solution will unilcr<,' 

 decomposition ; the metallic copper will be slowly and equally depo- 

 sited on the plate, and will form an exact counterpart or cast of 

 the form or pattern. It is the power thus obtained of copying in 

 metals deposited by electrolytic action any form or pattern which is 

 made the negative surface of a voltaic circuit, which constitutes the 

 basis of electro-metallurgy : and it now remains to explain briefly by an 

 example the mode of manipulation to be employed. 



Let the object to be copied be a small bas-relief of about six inches 

 by four, executed in a material such as marble, ivory, or plaster of 

 Paris, which will admit of a wax mould being taken from it by the 

 usual processes : to do this, the original, if of plaster of Paris, must 

 be well oiled at its surface, to prevent the wax from adhering to it ; 

 but the oil must be allowed to soak into the plaster : the wax, which 

 should be of the finest white kind, must be poured on, well melted, 

 and the plaster should be previously warmed to prevent the wax from 

 being suddenly cooled by the contact before'the whole surface is covered 

 with it : when cold, the mould may be detached from the original with- 

 out injury to either. That this mould may be employed as the conduct- 

 ing-plate of a voltaic circuit, its surface must be rendered conducting, 

 which the wax itself is not : for this purpose the surface is to be 

 covered over with the finest powdered plumbago, equally and lightly 

 rubbed in by means of a camel-hair pencil ; and the success of the 

 operation depends on the face of this mould being equally covered 

 with the plumbago, the back and edges remaining as they were. 



An earthenware or wooden vessel of any convenient form is chosen, 

 capable of holding the mould and a copper plate of the same size at 

 the distance of half an inch from it, so that they may be quite im- 

 mersed in the solution of sulphate of copper : it is better if the mould 

 and plate be vertical in the vessel. The copper plate must have a wire 

 soldered to its upper edge, for the purpose of connecting it with that 

 of the battery, both plates of which should consequently be provided 

 with binding-screws, to allow of these connections being made and 

 broken at pleasure. The wax mould must also be connected with the 

 zinc element of the battery by an intermediate wire, and cai e must be 

 taken that this wire be in perfect contact at the point of union with 

 the conducting surface on the wax mould ; while it obviously must 

 not be so connected as to injure that surface or disfigure it : the 

 plumbago accordingly must be accumulated round the part where the 

 wire is put into the edge of the mould, and continued to the front in 

 a narrow well-defined band. 



The mould and the plate being placed parallel and very near to each, 

 other, face to face, but not in contact, they must be retained in that 

 position hi any convenient manner ; but care must again be taken in so 

 doing that there is no contact by any conductor between them. The 

 vessel may then be filled with the saturated solution of sulphate of 

 copper and the wires connected with the battery, the precise size and 

 form of which are immaterial. The whole must now be left undis- 

 turbed for from twenty-four to thirty-six hoius, at the end of which 

 time, the mould, being detached from the battery and withdrawn, will 

 be found covered over with pure bright metallic copper, rough en the 

 outer surface ; but when separated from the wax by gently heating, it 

 will, if the operation has been successful, present a perfect copy of the 

 bas-relief, every line of which, to the most delicate markings, will be 

 found transferred to the metal with more precision and delicacy than 



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