478 



Mr. Warren De La Hue 



[Jan. 21, 



platinum, magnesium, and graphite, all give, under similar circum- 

 stances, precisely the same length of spark : aluminium, however, 

 gives a spark longer in the proportion of five to four. 



Before the spark jumps and the arc forms it is preceded by what 

 we have called the streamer discharge. This is different in appearance 

 at the positive and negative terminals. You are not able to see the 

 characteristics now that I produce the streamer discharge before you, 

 but they are represented enlarged on the diagram ; the terminals being 

 supposed to be a point and a disc, and the point being made alter- 

 nately positive and negative. When the point is positive the discharge 

 takes the form of a series of twisted streamers (Fig. 14), when 

 negative it is in the form of a brush (Fig. 15). The current which 



Fig. 15. 



takes place in the form of streamers at a distance but a very little 

 beyond the true striking distance, namely, that at which the arc is 

 formed, is only the -^-^^-^ part of the current which passes with the arc, 

 and this is only one-half of that of the battery when short-circuited. 

 When the streamer discharge is examined in a rotating mirror with a 

 microscope suitably constructed, it is seen that the negative current is 

 much the more continuous, for with the same velocity of the rotating 

 mirror the positive discharge breaks uji into a series of distinct images, 

 whilst the light of the negative is spread out into a sheet, as you will 

 see on the diagram (Fig. 16). The above effects were produced with 



