286 Prof. A. P. Chattock and Mr. A. M. Tyndall on the 



or diminishing the current strength, and it was there that 

 the field was measured in each case. 



The hysteresis is perhaps connected with the fact that 

 when P begins to discharge it sends + ions to N. These, 

 by rendering the escape of corpuscles easier (see below), may 

 increase the average ionizing power of the negative ions 

 sent to P and so diminish the field necessary for glow, and 

 therefore indirectly the current. 



Ageing of the Point. 



While external ions appear to exercise little influence 

 upon negative discharge from a new point the case is different 

 for an old one. It is well known that when a point has 

 been used a good deal it "ages'" for negative discharge by 

 requiring, not only a higher field to keep a given current 

 flowing from it, but a field which fluctuates widely ; the 

 ageing having apparently no effect upon positive discharge 

 from the same point. 



In the first set of experiments thirty curves were obtained 

 altogether for positive and negative discharge with and 

 without N, and by the end of the nineteenth the point 

 showed signs of ageing. This appears from Curves VI. 

 (PI. IV.), where the unconnected dots and circles represent 

 discharges from a negative point against a plate without N. 



Those observations made before the nineteenth curve are 

 marked by the do<s, and if joined up by lines give curves 

 that are more or less smooth ; but the circles which mark 

 the later observations give curves which zigzag up and 

 down in the most irregular way if treated similarly. 

 Instances of this irregularity are shown in Curves VII., 

 where are plotted the twenty-first and twenty-eighth curves 

 taken for negative discharge without N. 



If, however, external ions are supplied to the point the 

 irregularity vanishes. This is illustrated by the twenty- 

 fourth curve, also plotted in Curves VIL, which was taken 

 with N at distances x=l'b, y = 1*6 cm. 



In these earlier observations, as in the later ones of 

 Curves II., those for which external ions were supplied give 

 curves which are in close agreement with one another. The 

 mean position of the earlier of these curves is shown by 

 the line A A in Curves VI., and may be said to follow 

 approximately the line of dots, It is true that at small 

 currents A A is appreciably above the dots, as it is above 

 the lowest dots throughout its length, but if we allow for 



