DISRUPTIVE DISCHARGE — DIFFERENCE OF POSITIVE AND NEGATIVE BRUSH. 121 



ences, and they seem to show at once, that the negative surface tends to retain its 

 discharging character unchanged, whilst the positive surface, under similar circum- 

 stances, permits of great variation. 



1468. There are several points in the character of the negative discharge to air 

 which it is important to observe. A metal rod. OS of an inch in diameter, with a 

 rounded end projecting into the air, was charged negatively, and gave a short noisy 

 brush (fig. 8.). It was ascertained both by sight (1427- 1433.) and sound (1431.), 

 that the successive discharges were very rapid in their recurrence, being seven 

 or eight times more numerous in the same period, than those produced when the rod 

 was charged positively to an equal degree. When the rod was positive, it was easy, 

 by working the machine a little quicker, to replace the brush by a glow (1405. 1463.), 

 but when it was negative no efforts could produce this change. Even by bringing 

 the hand opposite the wire, the only effect was to increase the number of brush dis- 

 charges in a given period, raising at the same time the sound to a higher pitch. 



1469. A point opposite the negative brush exhibited a star, and as it was approxi- 

 mated caused the size and sound of the negative brush to diminish, and, at last, to 

 cease, leaving the negative end silent and dark, yet effective as to discharge. 



1470. When the round end of a smaller wire (fig. 9.) was advanced towards the ne- 

 gative brush, it (becoming positive by induction) exhibited the quiet glow at 8 inches 

 distance, the negative brush continuing. When nearer, the pitch of the sound of the 

 negative brush rose, indicating quicker intermittences (1431.) ; still nearer, the posi- 

 tive end threw off ramifications and distinct brushes ; at the same time, the negative 

 brush contracted in its lateral directions and collected together, giving a peculiar 

 narrow longisli brush, in shape like a hair pencil, the two brushes existing at once, 

 but very different in their form and appearance, and especially in the more rapid re- 

 currence of the negative discharges than of the positive. On using a smaller positive 

 wire for the same experiment, the glow first appeared on it, and then the brush, the 

 negative brush being affected at the same time ; and the two at one distance became 

 exceedingly alike in appearance, and the sounds, I thought, were in unison ; at all 

 events they were in harmony, so that the intermissions of discharge were either iso- 

 chronous, or a simple ratio existed between the intervals. With a higher action of 

 the machine, the wires being retained unaltered, the negative surface would become 

 dark and silent, and a glow appear on the positive one. A still higher action changed 

 the latter into a spark. Finer positive wires gave other variations of these effects, 

 which I must not allow myself to go into here. 



1471. A thinner rod was now connected with the negative conductor in place of 

 the larger one (1468.), its termination being gradually diminished to a blunt point, 

 as in fig. 10.; and it was beautiful to observe that, notwithstanding the variation of 

 the brush, the same general order of effects was produced. The end gave a small 

 sonorous negative brush, which the approach of the hand or a large conducting 

 surface did not alter, until it was so near as to produce a spark. A fine point oppo- 



MDCCCXXXVIII. R 



