
36 IONIZATION THEORY OF GASES [CH. 
29. ‘Townsend? has shown that, for low pressures, the variation 
of the current with the voltage is very different from that observed 
at atmospheric pressure. If the crease of current with the voltage 
is determined for gases, exposed to RGntgen rays, at a pressure of 
about 1 mm. of mercury, it is found that for small voltages the 
ordinary saturation curve is obtamed; but when the voltage 
applied increases beyond a certain value, depending on the pressure 
and nature of the gas and the distance between the electrodes, the 
current commences to increase slowly at first but very rapidly as 
the voltage is raised to the sparking value. The general shape of 
the current curve is shown in Fig. 5. 
Current 

The portion OAB of the curve corresponds to the ordmary 
saturation curve. At the point B the current commences to 
increase. This increase of current has been shown to be due to 
the action of the negative ions at low pressures in producing fresh 
ions by collision with the molecules in their path. The increase of 
current is not observed in air at a pressure above 30 mms. until the 
P.D. is increased nearly to the value required to produce a spark. 
This production of ions by collision is considered m more detail in 
section 41. 
1 Phil. Mag. Feb. 1901. 
