Specific Velocities of Ions in the Discharge from Points. «S0 



for both curves. There is thus no evidence that the negative 

 curve has been shifted bodily along z to anything like the 

 extent required; and this renders it very improbable that the 

 gas is ionized in the region of the point to an appreciable 

 distance from it. 



The only alternative seems to be a back-discharge from 

 the ring into the gas ; and as this is also consistent with the 

 shape of the curve, we shall assume that it occurs in the case 

 of negative discharge through hydrogen. 



Coming now to the fluctuations in V, it looks at first sight 

 suspicious that it is precisely in the case in which back-dis- 

 charge is high that these fluctuations are so marked; for if 

 the ions of the back-discharge travel as far as the part of the. 

 tube in which the pressure measurements are made, changes 

 in the amount of this discharge must give rise to apparent 

 changes in V. 



This, however, would mean that dp/dz must increase as z, 

 increases; whereas inspection of the negative curve for 

 hydrogen shows that for points beyond = 1*8 centimetre the 

 curve is straight. We may therefore conclude that in the 

 region beyond that distance from the surface of the ring 

 back-discharge ions are not present in any appreciable 

 number. 



Errors of observation being also insufficient to account for 

 the apparent changes in V, the possible explanations of these 

 thus reduce themselves to 



a. Fluctuations in the back-discharge outside the region of 



pressure measurements. 



b. Real changes in the value of V. 



Now fluctuations in the back-discharge must affect p l and 

 p 2 about equally on the average, and they will therefore pro- 

 duce the same sort of effect on V as the accidental errors of 

 observation. It is true that the amount of the back-discharge 

 may depend, for a given current, on the field at the surface 

 of the ring ; and it might be objected that, as this field is 

 greater for p 1 than for p^ owing to free electricity in the tube, 

 Pi should be more reduced than p%. But if fluctuations in 

 the amount of this free electricity are to account for changes 

 of 50 per cent, and more in V, the free electricity itself 

 must produce a far greater effect in increasing the apparent 

 value of V; from which we should have to conclude that our 

 value of V— is too high, the true value falling considerably 

 below that of V + . This would be contrary to the results 

 obtained by other observers however, and the objection thus 

 falls to the ground. 



At the same time it is quite possible that our value for V — 



