86 Dr. Norman Campbell on, 



It is to be observed that the effect of the collisions with the 

 molecules of the gas is to increase both the first and the third 

 terms on the right hand of the equation. 



When the gas is one which has no affinity for electrons 

 and reflects rather than absorbs them, the effect of collisions 

 is somewhat different. It is now possible that a 8 ray 

 starting from one electrode may be reflected back to that 

 electrode ; the number of such 8 rays crossing the condenser 

 is diminished. Further, since there is little loss of energy 

 at reflexion, and since the reflecting power of the molecules 

 does not change rapidly with the speed of the electrons, the 

 proportion of the electrons starting from one electrode which 

 are deflected back to it will he nearly the same whether they 

 are emitted with or against the field, so long as the potential 

 difference between the electrodes is small compared with the 

 velocity of emission. The collisions with the molecules wdll 

 diminish the part of the current due to the 8 rays from the 

 electrodes for such small potentials, while in the case of 

 unelastic collision they increased it. On the other hand 

 collisions, even if perfectly reflecting, will not diminish the 

 current carried by the Srays from the gas, for these electrons 

 must arrive ultimately at one of the two electrodes and 

 cannot cease to act as carriers by being reflected to the place 

 where they were generated. The current carried by these 

 ■8 rays depends on the ratio of those which arrive ultimately 

 at the oppositely charged electrode to those which arrive at 

 the similarly charged electrode ; reflexion cannot decrease 

 this ratio ; it will increase it if there is any loss of energy at 

 reflexion. Accordingly the third part of the current will be 

 increased whether the collisions are elastic or unelastic, but 

 to a greater extent in the second case. 



The second part of the current is independent of the number 

 of collisions, and in any case it is always very nearly saturated. 



I, Let us now examine the experimental facts in the 

 light of the foregoing discussion. Some apparently strange 

 results of measurements of the current through ionized air 

 and hydrogen at low pressure were described in a former 

 paper * ; they are now set out rather more fully in Tables I. 

 and II. The distance between the electrodes as well as the 

 pressure of the gas was varied : both quantities are expressed 

 by the ratio l/\, where lis the distance between the electrodes 

 and A, the calculated free path of an electron in the gas ; 

 this ratio is given in the top row. The second row gives the 

 value of n/N. T$e is the saturation current at zero pressure; 

 n cannot be found by direct measurements of the saturation 

 * Phil. Mag. Oct. m2 :V . oi>7. 



