288 Prdf. A. P; Chattock and Mr. A: Ms Tyndall on the 



It explairis, what appears to be the case from Curves VI. 

 and VII., that ageirig has less effect on the field at large 

 currents than at small ones. 



It explains the fact that discharge will not start as a rule 

 from an aged point lintil its electrification is fat* in excess of 

 what is required when the point is new. For this starting 

 of discharge depends on the presence of initially formed 

 positive ions ; ahd if, as these are very few in number, they 

 fail to obtain corpuscles from the metal, their only alter- 

 native will be to ionize the gas, which of course means a 

 higher field. 



Lastly it explains the effect of external ions on an aged 

 point. When discharge starts in the' mailner just described, 

 by ionizing the gas, the region on the point at which it takes 

 place will be determined by geometrical conditions alone, 

 and will therefore have nb particular connexion with the 

 place where corpuscles come Out most easily. It will, in 

 fact, tend to be the place where the point has been most 

 used, and therefore where they come out with greatest 

 difficulty, so that even when the current is well started and 

 the supply of positive iohs sufficient to obtain corpuscles 

 from the point we may still expect an abnormally high field 

 there; 



Now allow the initially formed ions to be reinforced by 

 supplies of external ions sufficiently large to knock out the 

 Corpuscles freely. It will no longer be necessary to raise 

 the field to that required for ionizing the gas before 

 discharge will start, as the conditions for Ordinary negative 

 discharge will obtain. But whereas when discharge was 

 started by initially formed ions alone it tended to occur at 

 an aged place on the point, it now starts at the place where 

 corpuscles come out easiest, since a large area of the point 

 surface is bombarded by the external ions, and the unaged 

 spots upon it are therefore sure to be discovered. The point 

 should consequently behave like a new one, and this, as the 

 experiments show, is precisely what happens. 



Our somewhat arbitrary assumption, that a small supply 

 of positive ions is prevented by the ageing change at a point 

 from bombarding corpuscles out of it, while a large supply 

 is not-, thus seems to fit the facts fairly well. 



In time of course a point ought to become aged all over 

 if persistently supplied with external ions. We do not know 

 whether this happens or not, but it is possible that the 

 beginnings of the process are to be seen in those curves of 

 the first set which were taken for negative discharge with 



