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



the field at the surface of the plate is increased, owing to the 

 increased quantity of free electricity in the discharge-tube. 

 It may well be that this entails an increase in the back 

 discharge when the latter is present, and a consequent further 

 decrease in the wind pressure. If this is so the apparent 

 increase in V — , while directly dependent upon back discharge 

 alone, may be indirectly dependent through it upon two dis- 

 tinct causes : one the cause of the back discharge itself, and 

 the other the cause, just referred to, which renders the back 

 discharge greater at large than at small values of z. 



In the 1901 work on hydrogen large variations were met 

 with in the calculated values of V— which could not be 

 attributed to errors of observation, and which seemed to 

 imply real changes in V— . At the time it was suggested 

 that the intermittent escape from the point of occluded gases 

 other than hydrogen might be responsible for the effect. In 

 the light of the present experiments, however, it is clear 

 that changes in the back discharge due to very small changes 

 in the amount of oxygen present are capable of having pro- 

 duced even larger changes in the value of V — than those 

 which were actually found. As such small changes in the 

 purity of the hydrogen must certainly have occurred in this 

 early work, it seems safe to replace occluded gas by variation 

 of back discharge in explaining the variations of V — . 



Heating Effect of Discharge. 



In our 1908 paper it was shown that if point discharge 

 occurs in a closed vessel the rise of temperature of the gas 

 depends on the specific velocity of the ions ; the rise per watt 

 of electrical energy supplied being a simple function of V 

 and increasing with it. 



The meaning of this is that if V be increased from any 

 cause, the ions by passing through the gas more rapidly 

 will exist at any moment in smaller numbers between the 

 point and the plate. The drag on the gas and consequently 

 the wind will be less. Hence the cooling will be less, and a 

 higher final temperature will be reached. 



Abnormally high temperature rises were recorded in the 

 case of negative discharge through nearly pure hydrogen 

 (loc. cit. pp. 44-47); and this was the reason we thought, as 

 mentioned above, that V— might show a corresponding 

 increase above its value in less pure gas. 



But back discharge will also lead to rise of temperature by 

 reducing the wind, and we now find that a reduction of the 

 oxygen present promotes back discharge when the point is 



