88 THE THEOEY OF IONIZATION OF GASES 



free from the negative electrode when positive ions 

 impinge on it. This additional process of ionization 

 would obviously be greater when the force is concen- 

 trated near the cathode than when the larger force 

 is at the anode. It is obvious, therefore, that when 

 these three processes of ionization are acting alone, the 

 potential required to give a spark or maintain a discharge 

 should dimmish when the force at the cathode increases, 

 so that the results obtained from the general formula for 

 sparking potentials could not be correct at the lower 

 pressures. 



In such cases it is necessary to introduce some impor- 

 tant factor in addition to the direct effects of collisions, 

 and a system which would account for the phenomena 

 has been suggested in section 32. It would be easy to 

 find mathematical expressions, based on various assump- 

 tions, to express the sparking potentials under these 

 conditions, but it is hardly desirable to make further 

 investigations of sparking potentials at low pressures 

 until some experimental evidence can be brought forward 

 in support of the principles that are involved. 



BRADBCJRY, AGNEW, & CO. LD., PRINTEES, LONDON AND TONBRIDGE 



