March 14, 1878] 



NATURE 



385 



ELECTRICAL ANALOGIES WITH NATURAL 

 PHENOMENA ^ 



II. 

 pOLAR AURORA.— ThQ experiments of De la Rive 

 ■'■ have already shown the connection of polar aurorae with 

 terrestrial magnetism ; but they do not explain all the 

 circumstances which accompany aurorae. In M. Plant^'s 

 experiments the electric current, in presence of aqueous 

 vapour, yields a series of phenomena altogether analogous 

 to the various phases of polar aurorae. 



If the positive electrode of the secondary battery is 

 brought into contact with the sides of a vessel of salt water, 

 we observe, according to the distance of the film from the 

 liquid, either a corona formed of luminous particles 

 arranged in a circle around the electrode (Fig. 8), an arc 

 bordered with a fringe of brilliant rays (Fig. 9), or a sinuous 

 line which rapidly folds and refolds on itself (Fig. 10). 

 This undulatory movement, in particular, forms a complete 

 analogy with v^hat has been compared in aurorae to the 

 undulations of a serpent, or to those of drapery agitated 

 by the wind. The rustling noise accompanying the ex- 



FiGs. 8, 9, 10. — Coronas and luminous arcs. 



periments is analogous to that sometimes accompanying 

 auroras ; it is caused by the luminous electric discharge 

 penetrating the moisture. As in aurorae, magnetic per- 

 turbations are produced by bringing a needle near the 

 circuit, the deviation increasing with the development of 

 the arch. Auroras are produced by positive electricity ; 

 the negative electrode produces nothing similar. 



Globular Lightning. — To study the effects produced 



Fig. II. — Flame produced over distilled water by an electric current of high 

 tension. 



on distilled water, M. Plants increased the tension of the 

 current, combining twenty secondary batteries, composed 

 each of forty couples, and forming a total of 800 second- 

 ary couples, whose current of discharge was nearly equal 

 to that of 1,200 Bunsen elements. 



When the current of this combination of batteries is 

 made to act on distilled water, he finds, first, in much 

 greater intensity, the effects already observed by Grove, 

 by means of 500 elements of his nitric acid pile. The 



» Continued from p. 229. 



positive electrode being inserted in the distilled water, 

 he obtains, by approaching the negative platinum wire to 

 the surface of the water, and immediately raising it, a 

 yellow flame, almost spherical, of about two centimetres 

 in diameter (Fig. ii). The platinum wire, two milli- 

 metres in diameter, melts ; the flame is formed by the 

 rarefied incandescent air, by the vapour of the metal of 

 the electrode, and by the elements of the vapour of water ; 

 spectral analysis shows clearly the presence of hydrogen. 



If, to avoid the fusion of the metal, we diminish the 

 intensity of the current by interposing a column of water 

 in the circuit, the spark appears under the very compact 

 form of a small globe of fire from eight to ten millimetres 

 in diameter (Fig. 12). On raising the electrode a little 

 more, this globe takes an ovoid form ; luminous blue 

 points, whose number varies continually, arranged in 

 concentric circles, appear at the surface of the water 

 (Fig. 13). Rays of the same colour soon issue from the 

 centre and join these points (Fig. 14). At intervals the 

 rays take a gyratory movement, now in one direction, 

 now in another, describing spirals (Figs. 15 and 16). 

 Sometimes the points and the rays disappear all on one 

 side, and varied curves, formed by the movement of those 

 which remain, are figured on the surface of the liquid. 

 Finally, when the speed of the gyratory movement 

 increases, all the rays vanish, and only blue concentric 

 rings are seen (Fig. 17). The rings are found to be the 

 last term of these transformations which are very curious 

 to follow with the naked eye or with a telescope, and con- 

 stitute a veritable electric kaleidoscope. 



The production of these figures is explained by the 

 great mobility of the arcs or luminous threads which com- 

 pose the ovoid light, formed between the water and the 

 electrode. On examining with care this particular form of 

 spark, he finds that it is, in reality, a sort of voltaic brush 

 discharge, analogous to the brush discharge of static elec- 

 tricity, but more dense on account of the greater quantity of 

 electricity in play. These luminous threads being in a state 

 of continual agitation, the points at which they encounter 

 the surface of the liquid are constantly displaced, and 

 form the rays observed. Their gyratory movement 



