QUICKER THAN LIGHTNING. 317 



Leyden jar furnished a number of single sparks, each time the coil was 

 excited, the number varying between one and thirty, according to cir- 

 cumstances. The whole proceeding consumed an interval of time often 

 as great as one-fiftieth of a second ; that is, the jar loaded up and dis- 

 charged itself twenty or thirty times in that period. Prof. Rood found 

 the number of elements of the spark to vary with its length, the nature 

 of the electrodes, and the size of the jar. Short sparks are more com- 

 plex than long ones, small jars give more than large ones, and metallic 

 points a greater number than balls. The point to be determined was, 

 the duration of the several elements of the spark, and especially of its 

 quickest element. In one case of a discharge lasting the fiftieth of a 

 second, it began with an ordinary spark, followed by a pale-violet 

 light, lasting about one-sixtieth of a second, and then came a compact 



Fig. 5. 



Images of Spark drawn out. 



body of ten or twenty sparks, this last act continuing for about one 

 two-hundredth of a second. The results of the inquiry are thus stated 

 by Prof. Rood : " From the foregoing, then, it appears that, if a jar, 

 having a metallic coating of about one hundred square inches, be con- 

 nected, as above desci'ibed, with an induction-coil, its discharge will 

 be effected by a considerable number of acts, of which the first is by 

 far the most intense. Further, the metallic particles, heated up by 

 the first discharge to a white heat, almost instantly assume a lower 

 temperature, marked by a corresponding change from white to brown- 

 ish yellow ; and, as their temperature continues to fall, the tint changes, 

 in the case of brass electrodes, to green ; in that of platinum, to a gray 

 or violet-gray. These observations further demonstrated the fact that 

 four ten-millionths of a second is an interval of time quite sufficient 

 for the production of distinct vision." 



It was also shown that the first act of the electric explosion, repre- 

 sented by the white band, lasted through an interval of time so short 

 as to be immeasurable. It was proved that it could not occupy more 



