Kl.rXTltnMKTKK. 



K.LKCTKO-MiiTlVK MA< MINKS. 



t:t 



the brushes and oompoiition. All the parU which are to be \\hitv 

 in the impreauon are left uncovered by the paint, either by handling 

 the brush *u an to avoid then, or mooring the paint from them by a 

 cork (tump or other mean ; the kind of itroke and the thickness of 

 the paint are made to depend on the depth of the ahade in the picture. 

 When the picture ii fmiabed, it u coated with black lead, and exposed 

 to the electro-coppering prooees, by which a plate U produced fit for 

 working in the copper |4ate press, having the linos of the device 



marked in intaglio or aunl 



In the ratface-printing method, the paint or oompoiition U so laid 

 on a* to cause a aeries of hollow* in the electrotype deposit sufficiently 

 deep to prevent being inked by the printer'* inking roller ; in other 

 word*, all thoee part* which are to be black in the impression are left 

 untouched < m the plate a plan directly the reverse of the former. In 

 , order to aid the artist in producing the required effect, he use* a white 

 paint on a bUck ground ; the metal plate maybe blackened by hydro- 

 ulphim't of ammonia, or by other agents ; and n white paint may be 

 formed from a mixture of sulphate of lead, white wax, lard, and olive 

 oil. The mode of pencilling is entirely different from that adopted in 

 the former case, and, as would appear, more difficult ; for the artist has 

 to .tvnid with his pencil all the part* which are to form the device or 

 inked part in the impression. \\ hen the plate is finished it is coated 

 with black lead, and immersed in the sulphate of copper ; whereby a 

 plate is produced fitted for surface-printing. 



Minor improvements have been introduced in these curious branches 

 of art ; and specimens of pictures produced by these means have been 

 given in the illustrated newspapers. 



In the above article the reader will find an explanation of the 

 scientific principles of electro-metallurgy, with a sufficient amount of 

 detail to render the practice intelligible. More than this ought not 

 to be expected in an encyclopjedio article. Should more be desired, 

 we cannot do better than refer to a volume just printed by order of 

 the Commissioners of Patents, entitled ' Abridgments of Specifications 

 relating to Electricity and Magnetism, their generation and applica- 

 tions.' This volume contains between 800 and 900 pages. Referring 

 to those parts which relate to electro-coating, electro-deposition, &c., 

 we have hundreds of patents, and as many applications, from the 

 covering of a fish-hook to the metallising of a human corpse. There 

 are also patents for coating steel pens and pen-holders, the hulls of 

 ships and the ribs of umbrellas, for covering vitrified surfaces and for 

 ]rodiicing damascene inlaid or encrusted work, for making copper 

 tubes and vessels, mirrors or specula, and artificial teeth and gums, for 

 manufacturing the soles of boots and shoes, chairs, bedsteads, and 

 coffins, daguerreotype plates and gold thread, hinges, mantel-pieces, 

 mathematical instruments and projectiles, spoons and forks, wire 

 gauze ; for metallising fibrous materials, for all kinds of ornamentation, 

 fur protecting telegraph cables and sepulchral monuments, &c., ftc. ; to 

 say nothing of printing, engraving, and the various other applications 

 already described or referred to in our article. 



K I - KCTRO'METER. This term strictly applies only to instruments 

 adapted to measure electricity ; it has however been applied in a more 

 extended sense to those which only indicate the presence of that fluid ; 

 but these are more correctly denominated dectroKoptt. 



Of the former kind is the Balance of Tortion invented by Coulomb, 

 to which we have had occasion to refer in the articles ELASTICITY and 

 ELECTRICITY. The following is a description of this delicate instrument. 



A very fine metallic wire, or, which is better, a single thread of silk 

 taken from the cocoon, or, which is better still, a filament of spun class, 



Awjvl l,_. . . i . .1 i >. .*...< 



icn is aiiocnea toe ooay to be electrised, a* for 



instance a small boll of elder .pith ; at the top of the suspended string 

 there is placed a plate moveable with friction on a glass cylinder, in 

 which the thread is contained, by which any requisite torsion msy be 

 given to the thread, which U shown by an index on a micrometer 

 crew ; the body of the large cylinder which enclose* the needle is also 

 surmounted with a graduated brass circle. In electrical rxjiviim. 'nN 

 the index of the micrometer is on its division zero, and the plate u 



turned round so a* to bring the needle and pith-ball to the xero of the 

 graduated circle on the string. Again a second ball is attached to the 

 extremity of a fine isolating cylinder inserted in the apparatus BO that 

 both bulls may be in contact without pressure. The balls are then 

 electrised by communication with some isolated and electrised body, 

 and acquiring similar electricities repulsion immediately take* place. 

 That attached to the needle being moveable with it, carries it round 

 through a certain angle, and after some oscillations settle* at a definite 

 position with respect to the fixed ball, thin angle being indicated by the 

 lower graduated arc ; the elastic force of torsion is then in equilibrium 

 with the moving force of repulsion between the balls, and hence a 

 measure of the latter can be obtained. In such experiments only a 

 very small electrical charge i* communicated to the bolls. 



('.miotnb, in seeking the law of electrical action, found that in the 

 first instance of his experiment the needle deviated by 86. Then, 

 communicating a torsion to the thread in a direction tending to dii 

 this deviation, he found that the micrometer index traversed 1 . 

 reduce the angle of deviation to 18, and 667* degrees of torsion was 

 necessary to bring it to 84* ; the thread being twisted by force* 

 applied at both ends, it is evident that the entire torsions in the two 

 latter coses are 126+ 18 = 144 and 567 + 84* = 57SJ, while in the 

 first case it is only 86*. By comparing the deviations with the torsions, 

 it was easily seen that the force of repulsion varied inversely as the 

 square of the distance between the bolls. It should be remember..! in 

 such experiments, that if the torsion of the thread be too great it* 

 elasticity will act imperfectly, and be no longer proportional to the 

 angle of torsion. [ELASTICITY.] 



In like manner the law of attraction of differently electrised ball* 

 was ascertained, the torsion being then employed in resisting the 

 attraction. We may observe here that the result* thus deduced are 

 necessarily approximative, and not exact, because the neutral elec- 

 tricity of the balls being partly decomposed by the mutual influence of 

 the electricities communicated, the small forces thus arising interfere 

 with the actions which should be due to the latter only. The attractive 

 and repulsive forces may also be estimated by disturbing the needle a 

 little from its position of equilibrium, and observing the number of 

 oscillations which it make* in a given time, as was adopted by l'.i> < in 

 determining the law of electro-magnetic action of a galvanic current. 



The proof-plane also used by Coulomb was merely a small disc of 

 gilt paper fastened to on isolating handle ; this he employed to <1 i 

 the distribution of electricity on the surfaces of bodies by touching 

 them with the plane at various points, and observing by means of the 

 torsion-balance the quantity of electricity taken up by contact, which 

 he assumed to be proportional to the quantity of electricity at tin- 

 point touched. 



Various instruments hove been constructed for estimating approxi- 

 mately the total quantity of electricity in the charge of on electrised 

 body, such a* Lane's, Henley's, and Cuthbertson's electrometers. 

 Henley's quadrant electrometer is represented under ELECTRICITY. 

 The most precise instruments of this description are those invented by 

 Harris : a description will be found in his 'Rudimentary Electricity,' 

 under the names of ' Quadrant Electrometer of Double Repulsion,' 

 the ' Bifilar Balance,' the ' Hydrostatic Electrometer,' the ' Scale-beam 

 Electrometer,' the last of which is represented under ELECTRICITY. 



Electroscopes indicate the presence of very small quantities of elec- 

 tricity, and therefore are generally used with a condenser; as in the 

 case of the gold-leaf electroscope, consisting of two small port i 

 gold-leaf laid flat together ; and when made to communicate by a con- 

 ducting stem, with a condenser which has acquired electricity from a 

 very feeble source, they diverge from each other. They have been 

 also employed to indicate atmospheric electricity. The gold-leaf elec- 

 troscope is represented under ELECTRICITY. 



Instruments have also been constructed for the purpose of indicating 

 the existence 'of electrical currents, a description of which will be found 

 under OALVANOMETKR, THERMO-ELECTRICITY, RHEOSTAT, &c. 



KI.KCTRO-MOTIVE MACHINES. The various rotatory motions 

 produced by the electro-magnetic force, and the enormous attractive 

 power developed by electro-magnets have led some ingenious mechanists 

 at various times to seek in electricity a motive power which . 

 thought would rival even steam itself. A common method of con- 

 structing an electro-motive machine WOK to connect a set of armatures 

 like the spokes of a wheel, so as to revolve horizontally round a point 

 situated midway between the poles of a horse-shoe electro-magnet. 

 In such cage the opposite ends of each armature would be attracted by 

 the pole nearest to it, and if the name of that pole were suddenly 

 changed the some armature would be as suddenly repelled. In tin, 

 way each spoke would be drawn under the poles of the electro-magnet, 

 and immediately driven away with equal force, and the momentum 

 caused by its approaching by attraction, would cause it to pass beyond 

 the attracting pole, an effect which would be assisted by the substituted 

 repulsion. In this way a rotatory motion of the wheel could easily 

 be produced. Unfortunately for so promising a result, these rot 

 have no permanent momentum, and arc incapable of overcoming 

 resistance or doing any work. It is true that an enormous weight may 

 be suspended from soft iron by a very small outlay of chemical force, 

 but there is so rapid n diminution of the inductive force by distance, 

 the magnetic attraction diminishing according to the law of the 

 inverse square of the distance of the surface of the magnet and 



