1881. J 



on the Phenomcnn of the Elcclric Dl>irhar<je. 



469 



I can let you sec tlio arc, altliou^li I am unable to show the 

 details of its structure ; thus, when 1 move the discharging key 

 the arc passes between the two points 0-7 inch apart fixed in 

 the micrometer-discharger (Fig. 0), in which, however, the terminals 

 shown consist of a point and disc, instead of two points, which I am 

 now using. And I may mention that before tlie discharge takes 

 place there is neither condensation nor dilatation of a gaseous nu'dium 

 in contiguity with the charged terminals, as has been suggested, 



Fig. G. 



whatever may be their form. The length of the arc varies with the 

 potential of the battery, and with the form of the terminals; between 

 points, the length of the striking distance increases as the square of 

 the number of cells employed. Thus, with 1000 cells the striking 

 distance is 0'0051 inch; with 11,000 cells it is 0*62 inch, as shown 

 in the diagram (Fig. 7). The potential of 11,000 cells put our 

 means of insulation to a severe test, and 14,400 cells overcomes it to 

 such an extent as to interfere so seriously with the striking distance 

 that I only obtain a spark • 7 incli long. 



