54 THE THEOBY OF IONIZATION OF GASES 



passes through the gas, and the potential difference at the 

 electrodes falls to R' B. 



The following example of an experiment 1 made with 

 air at 4*31 millimetres pressure between parallel plates 

 at 8 millimetres apart illustrates the phenomena. The 

 gas insulated when a potential difference of 601 volts 

 was established between the electrodes, but a spark took 

 place when the potential was raised to 603 volts, and a 

 current of *0052 ampere was established, which was 

 maintained by a potential fall of 350 volts between the 

 electrodes, the remainder of the 603 volts being taken 

 up in sending the current through a large external 

 resistance. In this case the difference between the 

 sparking potential and the potential required to maintain 

 the current was 253 volts. 



22. Properties of pointed and cylindrical electrodes. 



When electrodes of different shapes are used, as 

 when a discharge takes place from a point to a plane, the 

 sparking potential depends on the direction of the force. 



For pressures above the critical pressure the smaller 

 potential is obtained when the point is negatively 

 charged, and for pressures below a certain value the 

 smaller potential corresponds to the opposite direction 

 of the electric force. 



Thus the variation of potential with the pressure may 

 be represented by a curve similar to P Q E when the 

 point is positive, and when the force is reversed the 

 corresponding curve has a relative position similar to 

 P' Q B'. The two curves intersect near the point 

 corresponding to the minimum sparking potential, which 

 varies according to the shape of the electrodes. 



1 Philosophical Magazine, December, 1904, 



