130 ELEMENTS OF STATIC ELECTRICITY. 



inside of C. Hence, when the switch is open, we have 

 the difference in the brush discharge already described. 



But as the higher charge of D continues to accumu- 

 late on its inside coating, the tension increases on the 

 electrode R, till the electricity finally bursts through 

 the resisting air from R to P ; producing the spark and 

 snap when the switch is closed, as already explained. 



The effect of opening the switch is to substitute for 

 this metal conductor, which has comparatively no 

 resistance, a portion of the base, which is of kiln- 

 dried wood, and offers high resistance. This retards 

 the current, producing the difference of phenomena 

 between the bright, instantaneous spark of white light, 

 with its sharp report, and the slow moving brushes of 

 violet light, with their hissing, spitting sounds; and 

 from this slow movement we are able to determine the 

 direction of the discharge, as already shown. 



The cause of the subdivision of the spark when the 

 switch is open next claims attention. It has been 

 shown that the discharge between the inside coatings 

 through the electrodes P and 72, and the intervening 

 air space, is dependent on the counter discharge 

 between the outside coatings, through the switch, when 

 closed, or, through the kiln-dried wood of the base, 

 when the switch is open. This discharge may be seen 

 by opening the switch, half an inch or more, so that 

 the resistance of the air is less than that of the wood. 

 We then have a bright spark below, simultaneous with 

 the spark above. But when the switch is opened so 

 that the resistance of the air is greater than that of the 

 wood, the discharge below takes place silently through 

 the wood, and we have above, the subdivided, colored 

 discharge already described. 



