THEORY OF ELECTRIC DISCHARGES 75 



by examining either the variations of the potential 

 difference between the plates for different currents or 

 the sparking potentials between conductors arranged so 

 as to produce a variation of the force along the path of 

 the discharge. 



If the supply of ions were maintained by the effects 

 of collisions at the lower pressures, the sparking potential 

 for a gas between parallel plates should continue to be 

 greater than the potential difference which maintains a 

 current, but experiments show that at the lower pressures ' 

 the potential increases with the current. 1 This might be 

 due to a rise of temperature of the gas and a correspond- 

 ing reduction in the number of molecules between the 

 plates, in which case the potential maintaining the current 

 would tend to increase in the same way as the sparking 

 potential increases when the pressure diminishes. This 

 suggestion has not yet been tested experimentally, so 

 that it is uncertain whether it affords a complete explana- 

 tion of the effects obtained. 



These phenomena are most probably related to the 

 point discharges for the lower pressures, and a theory 

 which would explain why sparking is produced more 

 easily from a positively charged point in a gas at a low 

 pressure than from the same point negatively charged 

 would apply also in some degree to the potential required 

 to maintain a current. 



32. Processes of ionization that may account for effects 

 obtained at low pressures. 



In these cases, as well as in the discharge between 

 cylinders, the lower sparking potential is obtained when 

 the greater force in the gas is in the neighbourhood of 



1 J. A. Brown, Philosophical Magazine, September, 1906. 



