438 A. W. Wright— Forms of the Mectrical Discharge in Air. 



influence on the nature of the phenomena observed. If the 

 poles of the machine are gradually separated, a point is soon 

 reached, at which the bright sparks appear very irregukrlv 

 or cease altogether, the polar interval being then usually some 

 four or five inches, but varying considerably with the circum- 

 stances of the experiment. Examination readily shows that at 

 such times a dark discharge is taking place, and shadows may 

 be produced in the same manner as when the condensers are 

 not used, but less readily, and with less distinctness. The dis- 

 charge now becomes very variable in character, passing from 

 the ordinary bright spark to the non-luminous form already de- 

 scribed, and all intermediate grades may be observed, according 

 to the energy of the jet, or the tension of the poles. 



Often an incomplete spark passes, consisting of two portions 

 issuing simultaneously from the poles, the middle portion being 

 quite dark. The two ends, in this case, are usually character- 

 ized by many branches, which are more numerous and longer 

 ill the part next the positive pole, and the whole exhibits a red- 

 dish color, and a feeble degree of illuminating power, as com- 

 pared with the perfect spark, while the detonation of the dis- 

 charge is correspondingly enfeebled. This is the case distin- 

 guished by Eiess as the " weak spark " {schwache Funh),* and 

 it is characterized by him as a distinct form of the discharge, 

 due to a diminution of the tension of the electricity. 



That under particular circumstances, or with a special ar- 

 rangement of the machine, such as to give electricity of lower 

 tension, the weak sparks are produced more frequently, is true, 

 but the fact that they are often formed when the potential ot 

 the machine is very high, shows that they are not dependent on 

 that alone, but rather upon the tension at the poles, at the 

 instant of discharge. In these cases, the poles being separated 

 but a few inches, a rapid jet-discharge takes place, with the pro- 

 duction of the phosphorescent glow above described. -Now 

 when this occurs, a silent discharge is going on through the air. 

 and this may be so energetic as completely to prevent the tor- 

 mation of the ordinary spark, and to lower the tension at the 

 poles very considerably, A variation in the rapiditv wi ^ 

 which the machine is worked, or the prevention or ^^^''^'^^^l^^ 

 of the jet by any means, may then raise the tension ot tb 

 poles sufficiently to determine the passage of the weak sparK., 

 or even of the ordinary bright ones. . ... 



It maybe supposed that, at the distance in question, t^^ 

 mutual inductive influence of the two poles is just suiiicienx ^^ 

 r.Qi,=o ^ local increase of tension there, to such a degree as 



enable the negative electricity to overcome the resistance oi 

 air, thus setting up a sort of current between the poles. 



