ELECTRICITY. 



827 



diameter. At each extremity there is a metallic cap 

 terminating in a spherical knob. Small circular pieces 

 of tinfoil are fastened on the surface of the tube, by 

 means of gum, at a small distance from each, and 

 arranged so as to resemble in form the thread of a 

 screw, as represented in the figure. When one end 

 of this is held so as to receive sparks from the 

 machine, and the other made to communicate with 

 the earth by being held in the hand, the sparks will 

 pass along it, and the whole line of tinfoil disks will 

 be illuminated. 



The longest and most vivid sparks are obtained 

 between two conductors having a rounded form, as 

 may be exemplified in a common electrical machine, 

 by presenting a metallic ball to that side of the prime 

 conductor which is furnished from the cylinder of the 

 machine ; a spark is immediately seen, of consider- 

 able length, resembling a long streak of fire, extend- 

 ing from the conductor to the ball. Often, when the 

 spark is very long, it is seen to have an angular or 

 zigzag course, exactly like that of a flash of lightning. 

 This irregularity is probably occasioned by the fluid 

 darting obliquely in its course to minute conducting 

 particles, as those of moisture, that are floating in 

 the air, a little removed from the direct line of pas- 

 sage. Electrical light differs in no respect from the 

 light obtained from other sources. Its brilliancy 

 depends upon the conducting power of the bodies 

 l>et\veen which it passes. When dry wood is employ- 

 ed, it appears in the form of faint red streams ; but 

 metals afford a light of greater brilliancy. Its colour 

 is subject to variations, from a great number of dif- 

 ferent circumstances. Sparks passed through balls 

 of wood or ivory are of a crimson colour ; but this 

 depends upon their position with regard to the sur- 

 face. Electric sparks, passing from one polished 

 metallic surface to another, are white ; but if the 

 finger be presented to an electrified conductor, the 

 sparks obtained are violet. They are green, when 

 taken from the surface of silvered leather ; yellow, 

 when taken from finely powdered charcoal ; and of a 

 purple colour, when taken from the greater number 

 of imperfect conductors. In exceedingly rarefied 

 air, the colour of the spark is green ; in denser air, 

 it acquires a blue tint, and passes to a violet and 

 purple as the condensation of the air is increased. 

 In making these experiments, it is found that in pro- 

 portion as the medium is more rare, its conducting 

 power increases, and a smaller intensity of electricity 

 is required for the production of light. In the ordi- 

 nary vacuum of the air-pump, the passage of electri- 

 city is rendered sensible by streams or columns of 

 diffused light occasionally varying in their breadth 

 and intensity, and exhibiting movements which give 

 them a marked resemblance to the coruscations of 

 the aurora borealis. It was at first imagined, that 

 the light which appears during the passage of elec- 

 tricity was actually the electric fluid itself, become 

 luminous from its high degree of accumulation. But, 

 since we know that common atmospheric air becomes 

 luminous by violent compression, and we must also 

 presume that electricity exerts a very sudden and 

 powerful pressure upon the air, by its passage 

 through that resisting medium, we are certainly 

 justified in drawing the inference, that the same 

 phenomena proceed, in both cases, from the same 

 cause. The sound, which accompanies the various 

 modes of transference, is subject to modifications de- 

 pendent upon the degree and suddenness of the im 

 pulses given to the air. The full, short, and undividec 

 ((park is attended with a loud explosion ; the more 

 lengthened spark, with a sharper snap, which becomes 

 more broken and rattling in proportion to the dis- 

 tance it has to traverse. The great increase o 

 intensity which the electric fluid acquires at tin 



xtremities of all elongated conducting bodies, and 

 specially the indefinite augmentation of this inten- 

 ity at the apex of all projecting points, has been 

 lluded to above. This intensity will necessarily be 

 ccompanied with a powerful disposition in the fluid 

 o escape a circumstance which furnishes a natural 

 nd exact explanation of the rapid dissipation of 

 lectricity, which takes place from all bodies of a 

 lender and pointed form. The illustration of these 

 ositions is seen in bringing metallic rods of different 

 orms near the prime conductor of a machine charged 

 vith either species of electricity, the conductor being 

 urnished with a pair of pith balls, suspended by a 

 me wire, whose divergence indicates the presence 

 nd degree of the electricity in the conductor : if the 

 metallic rod have a bell at the end which is brought 

 ear the charged conductor, the pith balls will be 

 iut slightly affected ; whereas, if it terminate in a 

 harp point, and the point be presented to the con- 

 uctor at the same distance as the ball was in the 

 ormer case, the divergence of the balls will immedi- 

 itely cease, showing that the electrical charge has 

 vholly disappeared. Currents of air always accom- 

 >any the discharge of electricity from pointed bodies ; 

 or each particle of air, as soon as it has received its 

 electricity from the point, is immediately repelled 

 jy the body. Many amusing experiments are 

 bunded on this principle. Let two cross wires, bent 

 at right angles near the ends, which terminate in 

 >oints, and pointing in a similar direction with respect 

 jo the axis, be supported at their centre upon a fine 

 aoint, and electrified by being placed upon the prime 

 conductor of a machine ; each of the points will give 

 )fl*a stream of electricity, and the wires will revolve 

 )ackward with considerable rapidity. An apparatus 

 consisting of wires terminating in points, and having 

 mils annexed to them, to represent the planets, 

 may be constructed so as to revolve when electrified, 

 and thus to imitate the planetary motions. Such an 

 apparatus has been called an electrical orrery. When 

 the transfer of electricity takes place between smooth 

 surfaces of a certain extent, no difference can be 

 jerceived in the nature and appearance of the spark, 

 vhichever be the position of the negative surface. 

 But in the passage of electricity through points, the 

 effect is considerably modified by the species of 

 slectricity with which the bodies are charged ; or, 

 in other words, by the direction in which the fluid 

 moves. When the electric fluid is escaping out of a 

 pointed conductor, the luminous appearance is that 

 of diverging streams, forming what is termed a 

 oencil of light, and resembling the filaments of a 

 brush. When, on the contrary, the electric fluid is 

 entering into the pointed body, the light is much more 

 concentrated at the point itself, having a resemblance 

 to a star, in which, if any streams appear, they are 

 disposed like radii, and equally so in all directions. 

 This difference in these two appearances may be em- 

 ployed, on many occasions, as a useful criterion of 

 the species of electricity, at least, which is passing 

 from one conductor to another, if not of the absolute 

 direction of its motion. For if a needle be presented 

 to an electrified body, the appearance of a star on the 

 needle will show that the electricity of that body is 

 positive ; while, on the contrary, a luminous brush 

 on the needle will indicate that the body is negative. 

 These observations seem to indicate the emanation 

 of some material fluid from the positive, and its recep- 

 tion by the negative point. It has, accordingly, 

 been urged, as an argument in favour of the Frank- 

 linian theory. The diverging lines on one side, and 

 their reflections on the other, represent exactly the 

 paths of particles flowing out as from a pipe, and 

 urged forward by a force which gives them such a 

 projectile velocity as to prevent their spreading cut 



