830 



ELECTRICITY. 



ltd the discharging rod is employed. It consists, as 

 shown above, of two wires, terminated at one end by 

 brass balls, and connected at the other by a joint, 

 which is fixed to the end of a glass liandle, and 

 which, acting like a pair of compasses, allows of 

 the balls being separated at different distances. 

 When opened to the proper degree, one of the balls 

 is made-to touch the exterior coating, and the other 

 ball is then quickly brought into contact with the 

 knob of the jar, and thus a discharge is effected, 

 while the glass handle secures the person holding it 

 from the effects of the shock. If we wish to send 

 the whole charge of electricity Uirough any particu- 

 lar substance, which may be tiie subject of experi- 

 ment, we must so arrange the connecting conductors, 

 as that the substance sliall form a necessary part of 

 the circuit of the electricity, as it is termed. With 

 this view, we must place it between two good con- 

 ductors, one of which is in communication with the 

 outer coating ; and the circuit may then be com- 

 pleted by connecting the other conductor with the 

 inner coating, by means of a discharging rod, to 

 one branch of which, if necessary, a flexible chain 

 may be added. 



VII. In forming arrangements for directing the 

 passage of accumulated electricity, it should be 

 borne in mind, that the electric fluid will, on these 

 occasions, always pass through the best conductors, 

 although they may be more circuitous in prefer- 

 ence to those which are more direct, but have infe- 

 rior conducting power ; and it must also be recol- 

 lected, that when different paths are open for its 

 transmission along conductors of equal power, the 

 electricity will always take that which is the short- 

 est. Thus, if a person, holding a wire between his 

 hands, discharges a jar by means of it, the whole of 

 the fluid will pass through the wire, without affect- 

 ing him ; but if a piece of dry wood be substituted 

 for the wire, he will feel a shock ; for, the wood 

 being a worse conductor than his own body, the 

 charge will pass through the latter, as being the 

 easiest, although the longest circuit. During its 

 transit through the human body, in like manner, 

 the shock is felt only in the parts situated in the 

 direct line of communication ; and if the charge be 

 made to pass through a number of persons, who take 

 one another by the hand, and form part of the circuit 

 between the inner and outer coatings of the jar, 

 each will feel the electric shock in the same manner, 

 and at the same instant ; the sensation reaching 

 from hand to hand, directly across the breast. By 

 varying the points of contact, however, the shock 

 may be made to pass in other directions, and may 

 either be confined to a small part of a limb, or be 

 made to traverse the whole length of the body, from 

 head to foot. By accurate experiments it appears, 

 that the force of the electric shock is weakened, i. e. 

 its effects are diminished, by employing a conductor 

 of great length for making the discharge. But it is 

 difficult to assign a limit to the number of persons 

 through whom even a small charge of electricity may 

 be sent, so that all shall experience the shock ; or 

 to the distance along which it may be conveyed by 

 good conductors. The abbe Nollet passed an elec- 

 trical shock Uirough 180 of the French guards, in 

 the presence of the king ; and the sensation was 

 felt at the same moment by all the persons compos- 

 ing the circuit. An experiment was made near 

 London, at a time when the ground was remarkably 

 dry, to ascertain if any loss of time accompanied the 

 passage of the fluid, when transmitted through con 

 siderable distances. It was made to perform a cir- 

 cuit of four miles ; being conducted for two miles 

 along wires supported on baked sticks, and for the 

 remaining distance through the dry ground. As %r 



as could be ascertained by the most careful observa- 

 ion, the time in which the discharge was transmitted 

 along that immense circuit was perfectly instantane- 

 ous. A retardation in the passage of electricity, 

 lowever, does take place, if the conductor be not of 

 i sufficient size ; and when this is the case, as 

 well as in those instances where the conductor is 

 not a good one, the discharge will not be effected 

 so instantaneously or so completely. Under these 

 circumstances, also, there is a tendency in the fluid 

 to diverge from the direct line of its course, and to fly 

 off to different objects in the vicinity, as is often exem- 

 plified in the case of lightning, which, on striking a 

 t>uilding, is apt to take a very irregular and seem- 

 ingly capricious route, darting towards conducting 

 bodies which may liappen to attract it, although at 

 some distance from the immediate direction it was 

 pursuing. The motion of electricity through perfect 

 conductors is attended with no perceptible alteration 

 in the mechanical properties of the conducting 

 bodies, provided they be of sufficient size for the 

 charge of the electric fluid transmitted. On the con- 

 trary, very considerable effects are produced when a 

 powerful charge is sent through a wire, which is too 

 small to allow the whole quantity to pass with perfect 

 freedom ; or through an imperfect conductor, though 

 of a large size, as is proved when a tree is struck by 

 lightning. A piece of dry writing paper, as well as 

 pieces of dry, porous wood, are easily torn in pieces 

 by an electric charge. 



VIII. Electricity exerts a most extensive and im- 

 portant influence in effecting changes in the tempera- 

 ture and chemical composition of bodies. The igni- 

 tion and fusion of metals by the electric discharge, 

 are phenomena which have been long observed. Thus, 

 by passing a strong charge through slender iron 

 wires, or the finest flatted steel, called pendulum 

 wire, they are ignited, and partly melted into globules, 

 and at the same time partially oxidated. If a 

 slip of gold or silver leaf be placed on white paper, 

 and a strong shock passed through it, the metal will 

 disappear with a bright flash, and the impulse with 

 which its particles are driven against the paper will 

 produce a permanent stain of a purple or gray colour. 

 The colours produced in this way have been applied 

 to impress letters or ornamental devices on silk and on 

 paper. For this purpose, the outline of the required 

 figure should be first traced on thick drawing paper, 

 and afterwards cut out in the manner of stencil plates. 

 The drawing paper is then placed on the silk or paper 

 intended to be marked ; a leaf of gold is laid upon it, 

 and a card over that ; the whole is then placed in a 

 press or under a weight, and a charge from a battery 

 sent through the gold leaf. The stain is confined, by 

 the interposition of the drawing-paper, to the limit of 

 the design, and in this way a profile, a flower, or 

 any other outline figure, may be very neatly impress- 

 ed. The heat evolved by electricity, like most 

 other of its effects, is in proportion to the resistances 

 opposed to its passage. A rod of wood, of consider- 

 able thickness, being made part of the circuit, has its 

 temperature sensibly raised by a very few discharges. 

 Most combustible bodies are capable of being 

 inflamed by electricity. Thus alcohol, ether, cam- 

 phor, powdered resin, phosphorus or gun-powder 

 may be set on fire. And the sparks taken from a 

 piece of ice are as capable of inflaming bodies as 

 those from a piece of red-hot iron. The oxidation of 

 metals, through which accumulated electricity fias 

 been passed, is rather to be ascribed to the tendency 

 which they are known to possess of combining with 

 the oxygen of the atmosphere when heated, than to 

 any peculiar agency of electricity. A reverse pro. 

 cess, however, is found to attend electrical discharges 

 through metallic oxides, extricating their oxygen, 



