454 Prof. Chattock and Mr. Tyndall on Pressure of the 



dp/dz is the slope o£ pressure for a current o£ 1 micro- 

 ampere from the point, and V is the ionic velocity in a field 

 of 1 volt-cm. -1 



Curves I. 



Tke. Curve marktei iu circZes 

 :S the same as tha.t -tha-yVc^ fcy 

 (Lots wttti Hie ]3e.Tcen^a.ge scale. 

 Ytvo^'hiVie.Z one k-urvirei foli — 



I I ~ I 



2. 3 rferemfcx^e Oxygen 



It was found that the values of dp/dz were not independent 

 of the current used to obtain them, and that they increased 

 as the current was reduced — the negatives considerably. 

 The current used for all but the very low percentages was 

 about 2 microamperes. For these, in the case of negative 

 discharge only, it was increased about three-fold, as the 

 pressures to be measured were then so \erj low. 



The numbers in the second column have all been corrected 

 to zero current. This meant an increase in the pressure 

 slope per microampere obtained from a current of 2 micro- 

 amperes of about 20 per cent, for negative and 5 per cent, 

 for positive discharge, in the case of the higher percentages. 



Now it is most unlikely that the negative ion travels at 

 anything like the speeds shown here — speeds which are 

 moreover out of all proportion greater than those suggested 

 by our 1908 work; and, in fact, we found later that it was 

 possible to obtain values of dp/dz which were actually though 

 very slightly negative for discharge in what was perhaps 

 still purer hydrogen *. 



* A little paraffin-wax had been discovered between the two sets of 

 experiments upon the glass under one of the mercury seals ; and this 

 may have given rise to a very slow leak of air into the apparatus between 

 the wax and the glass in the first experiments. 



