i:Y I!<>T 1'LATINTM IN DIFFERENT OASES. 



wliicli is put fijual to unity. The absolute values at different pressures can be 

 obtained from the results for the pressure variation which will be given in a later 

 section. 



200 300 



Volts 



Fig. 2. 



400 



These results can readily be explained if we suppose that the wire produces at its 

 surface a constant number of positive ions per second which, however, depend, as 

 will be seen later, on the pressure of the gas and these ions are all collected 

 unchanged by the electrode at low voltages. When the voltage increases to 200 or 

 more, these positive ions produce others by collision and an increase in the current is 

 obtained. The above increase in the current possesses all the features which are 

 ivijuired by the view that it is due to ionisation by collision. The change produced 

 by altering the P.D. from one to another assigned value increases as the pressure is 

 diminished, reaches a maximum and ultimately disappears. Thus the ratio of the 

 current under a given voltage to that under 80 volts reaches a maximum as the 

 pressure is diminished. 



The difference in the ett'ect of the positive and negative ions from hot metals in 



VOL. OCVII. A. C 



