480 



SCIENCE. 



Sparks produced by this machine attain, when fully 

 exposed to the air, a length of 12 centimetres when in- 

 fluenced by the secondary battery of 800 couples. With 

 less powerful machines, however, the length is reduced, 

 and, according to M. PUnte, it will be in proportion to 

 the number of condensers. When a spark discharges 

 across metallic filings it sometimes reaches the length of 

 70 centimetres. 



We must remark here that discharges produced in this 

 way have no alternate positive and negative sense, but 

 are always the same. The loss of force resulting from 

 the transformation should, therefore, be less than in in- 

 duction machines ; for as the Voltaic circuit is never 

 closed upon itself for a single instant, no portion of the 

 current is converted into a calorific effect. The machine, 

 moreover, can be kept rotating for a long time and it 

 produces a considerable number of discharges without 

 any apparent weakness being visible on the part of the 

 secondary battery. 



The most interesting effects studied by M. Plante, by 

 means of the sparks of this machine, were obtained by 

 causing them to pass over pulverized sulphur in a com- 

 pound of sulphur and minium. If these powders are 

 spread upon a surface composed of resin and parafnne 

 (1/10) the sparks, while passing over the sulphur, leave a 

 bluish line distinctly visible, as though traced in black 

 lead. This gives us an exact autograph, we may say, of 

 the spark's course. It is easily effaced, however, by 

 being rubbed. But if carefully followed and indented 

 with some sharp pointed instrument it can be rendered 

 intact. Afterwards we can study it thoroughly by 

 tracing a drawing of it. The sparks represented by Fig. 

 2 were produced as above described. 



When we come to investigate these sparks, we find, 

 according to M. Plante, that when they have not the 

 maximum of length which they are capable of attaining, 

 they often display enclosed branches resembling anasto- 

 moses, and which are likely to escape while our attention 

 is fixed upon the luminous track. Their sinuosities are 

 always rounded, and that angular zig-zag, which is 

 apparent in most electric sparks, is never seen. It is 

 true that this form is sometimes indicated by effects of 

 perspective when the flash is at the horizon. The sinu- 

 ous shape, however, predominates, and frequently the 

 spark resolves itself into two demi-undulations, forming 

 a sort of S, which is also often seen in flashes of light- 

 ning that strike the ground. We find there particularly 

 a very characteristic hook-shaped form, upon which M. 

 Plante has long endeavored to attract attention, and 

 which is produced at the negative pole in a constantly 

 varying manner. M. Piante thinks the formation of this 

 hook arises from the collision of two motions opposed to 

 the ponderable matter drawn from the points of the exci- 

 tant, an effect which always happens under an angle 

 more or less pronounced, and with a more rapid move- 

 ment on the side of the positive pole than the negative, 

 doubtless because there is greater electric tension at the 

 former. Our readers will probably recollect that I de- 

 monstrated this tension in several ways with the induced 

 currents of Ruhmkorff's bobbin. 1 



It a portion of the sulphur spread upon the plate of the 

 excitant is removed by giving the latter a few slight taps, 

 the sparks change to luminous branch-like aigrettes 

 which are truly magnificent. Fig. 3 represents one of 

 these at its natural size, having a luminous track 15 

 centimetres in length. M. Plante calls these arborescent 

 sparks, and he thinks they serve to explain those im- 

 pressions of a vegetable appearance sometimes observed 

 upon the bodies of persons struck by lightning, and 

 which merely result from the ramifications of the fiery 

 track made by the flash itself. He attributes these im- 

 pressions to certain pulverulent particles which are in 



the course of the discharge, and which, after being pro- 

 jected into the air, heated or blazing, in various direc- 

 tions, fall upon the body that has been struck and 

 produce a kind of cauterization if the particles are merely 

 heated, and luminous impressions if they are blazing. 



These experiments are extremely interesting, for they 

 clearly show that the pretended reproduction of neigh- 

 boring objects upon persons struck by lightning is purely 

 imaginary. 



It we neglect to give the taps, before mentioned, to the 

 sulphur-powdered plate, the spark is displayed as repre- 

 sented by Fig. 4. We observe, in this case, that the size 



Fig. 4. 



of the track is increased on the side of the positive pole, 

 and grows contracted as it advances towards the nega- 

 tive pole. Around the positive pole we see traces cor- 

 responding to branches or rays in proportion to the 

 quantity of sulphur remaved. On the side of the negative 

 pole we find circular tracks of an entirely different kind, 

 representing, probably, the luminous spots, generally 

 blue, which appear at the negative pole simultaneously 

 with the spark of Ruhmkorff's bobbin. 



If the plaque of the excitant belonging to M. Plante's 

 apparatus is arranged so as to produce Lichtenberg 

 figures— that is to say, covered with a compound of pure 

 powdered resin, pulverized sulphur and minium — magni- 

 ficent arborescent sparks of yet another kind can be 

 obtained, the most curious examples of which are shown 

 by Figs. 5 and 6. Tracings of these sparks are made by 

 placing a sheet of varnished black paper upon the plate. 



The different effects produced by the aigrettes and the 

 sparks are particularly marked. When the distance be- 

 tween the points of the excitant is too great to admit of 

 the spark discharging, and merely an aigrette appears, 

 the electric movement of ponderable matter which leaves 

 the negative pole and is manifested by the powdered 

 Itanium adhering to the resin, does not extend to the 

 positive pole. The latter presents no traces of red 

 powder in the sulphur wreath and divergent rays sur- 

 rounding it, as may be seen in Fig. 5. If the spark has 



1 See Vol. II. of La Lumiere Elcctrique p. 439, and also a paper on La 

 Horn homogintiti de Cc'tinceUt a" induct ion, p. 89. 



18111m 



MP 



Fir.. 5. 



discharged, however, the wreath is open and the interior 

 filled with red dust, showing that the electric movement 

 proceeding from the negative pole, extends to the de- 

 parting point of the positive electricity, as represented 

 by Fig. 6. 



