A. W. Wright— Forms of ihe JEkctrkal Discharge in Air. 439 



it is noticeable that after such a current is once established the 

 poles maj often be gradually separated to a much greater dis- 

 tance without interrupting it, though if, after this greater sep- 

 aration, the spark passes, or the poles are touched with a con- 

 ductor, the current is not readily re-established without ap- 

 proaching them to each other again. The analogy of this 

 circumstance, as well as of the curved foi-m of the lines which 

 the current often exhibits, with similar phenomena of the vol- 

 taic arc, is obvious. 



Riess has remarked (loc. cit.) that the weak spark mav be 

 regarded as the gerrn of the complete and bright sparks. " We 

 may go further, however, and say that the brush-discharge is 

 the germ of both the others, and indeed of most of the luminous 

 forms of discharge. When an elongated conductor, connected 

 with the machine and highly charged, is insulated, and so far 

 removed from any other conducting body that the attractive 

 influence of the latter is null, or may regarded as such, the 

 tension quickly rises to a maximum at the extremities, and, 

 if these terminate in blunt points or small balls, finally over- 

 comes the atmospheric resistance, and a discharge occurs in the 

 torm of a brush, or rather in that of a tree, the stem of which 

 IS toward the charged body. The ramified form of the dis- 

 charge IS evidently due both to the tension of the electricity, 

 ' ) the small electrical capacity of the au-. The portion of 

 * m contact with the pole becomes highly charged and is 

 ^Qen repelled, and a stream of such particles will follow in rapid 

 succession. Each of these particles communicates a portion of 

 Its charge to others which it touches, and so is in a condition 

 \ ^vv^^^® °^°^® electricity ft-om the charged body. Thus is 

 established, not merely a convective action in the particles of air, 

 out a so a current of electricity passing along the stream of 

 particles. The stream or thread of air is heated hj the passage 

 Of^the electricity, and all the more as its resistance is veiy great 

 • ^^^ *^e nseinthe temperature, however, its conducting power 

 li increased, and it thus not only conveys a larger quantity of 

 eiectncity, but becomes more highly charged, until finally, 

 ^^ng Itself as a highly charged body, it gives rise to other 

 ^';eams forming branches or lateral discharges, which in turn 

 L ^^^ *^ other ramifications, thus producing the familiar 

 •^oorescent forms of the discharge. 



hrn i!^ two conductors oppositely charged with electricity are 

 snffi • ?^ ^^^^ one another that inductive action becomes 

 "jV^ciently intense, we may have either the silent and non-lu- 

 ^ous discharge, or if the quantitv of electrity is too great to 

 ^ it'l equilibrium in this manner" the brush -discharge taking 



the air i 



P^cefro^ 



both poles simultaneously, and greatly 



»vfl,„- ^"^ poies simuiianeousiv, auu. gi'.a^v "" 



y ^^eir mutual action. The branches become longer, more 



