Mass of the Ions in the Electric Wind in Air. 



419 



of current passed the other way, as occurred when the cathode 

 was provided with a sharp point projecting slightly from its 

 surface, the field at the original (anode) discharging-point 

 might be lowered 20 or 30 per cent, or more. With the 

 electrification reversed, on the other hand, this was not the 

 case to anything like the same extent. 



Negative ions arriving at an anode thus greatly facilitate 

 discharge at the anode ; while positive ions arriving at a 

 cathode produce very little effect. I am therefore inclined 

 to attribute the negative value of it in curves 111, to a small 

 back-discharge from plate to point brought about by the 

 negative ions arriving at the plate. 



Miss W. E. Walker, who is at present engaged in applying 

 the " point and ring " method to non-conducting liquids, finds 

 very marked indications of a similar back-discharge in certain 

 liquids ; and, with them as with air, most particularly for 

 negative ions arriving at a positive ring. The same sort of 

 effect has also been met with in vacuum-tube experiments. 

 The whole question of back-discharge is very interesting, as 

 being likely to throw light on the mechanism of discharge ; 

 and I hope soon to publish an account of the facts upon which 

 the above statements are based. 



In the experiments with the thin ring the back-discharge, 

 if it occurred, would increase instead of decreasing the pres- 

 sure, as its direction would, on the average, be that of the 

 arrows, b (fig. 4), which obviously act the same way as the 



Fijy. 4. 



B 



arrows, a, of the direct discharge from the point. Error 

 would therefore still be introduced into the value of it, unless 

 b happened to be constant for both the points. Here, again, 

 there seems to be reason for doubting the accuracy of the 

 number for the negative cluster. 



A last source of error, common to both positive and nega- 

 2G2 



