of Steel Needle -Points in Air. 295 



second value of / is often slightly lower than the first ; as if 

 the film had not had time to finish forming again after the 

 first discharge. Now if this were so it would follow that the 

 value of f, when the current stops, should be less than when 

 it starts (for a used point only ; the two values agree closely 

 for a clean one). To test this needle H was again taken and 

 the values of / determined for various air-pressures at the 

 point when the current just stopped, the potential of the 

 needle being gradually diminished until a high-resistance 

 (7000 &)) Elliot galvanometer between the needle and the 

 earth stood at zero. The results are given in Curves III. ; 

 A B and C D for the unused point are repeated from Curves II. 

 for comparison. J K and L M are the new curves. They 

 are marked by circles. The agreement between J K and A B 

 could not be closer, and points unmistakably to a temporary 

 cleaning of the point while current is flowing. LM, for 

 negative discharge, is very curious — coinciding as it does 

 with C D at low pressures, while it leaves it so completely 

 higher up. It was noticeable that just in proportion as the 

 two curves diverged, the stoppage of the current was marked 

 by increasing suddenness. At high pressures it was impossible 

 to get the needle to discharge at all with a small current ; 

 either the flow was strong or it stopped altogether with a 

 jerk — the same effect, though almost microscopic, being just 

 perceptible at the cessation of positive discharge also. This 

 being so, it follows that the values of/ on the negative curve 

 really correspond to a strong current and not to the point 

 where the current stops. Hence their greater values. The 

 sudden cutting off of the negative current was very striking, 

 and suggested rather forcibly the covering over of the point 

 by some sort of film that had been perforated by it. It is 

 true that the positive discharge was hardly affected, but this 

 is only an example of what has been already alluded to — the 

 relatively greater instability of negative than of positive dis- 

 charge. 



Although the silent discharge from a point does not appear 

 to have the power of permanently clearing away surface- 

 resistance, sparks do so readily enough — though under suitable 

 circumstances they will also form it again. In making the 

 measurements on needle B (Table I.), the discharge at the 

 last recorded ( + ) reading took the form of a spark. V? 

 was here 1*59. On repeating the reading it had risen to 1'95. 

 A third discharge brought it back to 1*62, and after that it 

 continued to oscillate between these two values for some 20 or 

 30 times with hardly a break in the regularity. Obviously 

 one spark formed a resistance and the next blew it away 



