194 



Dr. B. Hodgson on Heating Effects at 



If now V is plotted against pressure with constant current 

 the curves shown in fig. 3 are obtained. They show also the 



Fie-. 3. 



relation between pressure and energy received per second^ 

 the scale being in this case different for each curve. Five 

 additional curves, similar in shape, can be got from Tables 

 III., IV., and V. They all show a pressure range from 2 to 

 4 mm. in which the energy given to the anode is approxi- 

 mately constant. As the pressure is reduced the energy per 

 ion increases to a maximum at about 1 ; 5 mm. pressure. It 

 then falls to a minimum whose position and value vary with 

 the current, and after this minimum a rapid rise is obtained 

 for lower pressures. 



§3.2 heoretical Conclusions. — I. The Large 

 Heating Effect. 



From Table I. it will be seen that the energy com- 

 municated to the anode per second for a current of 0*003 

 amp. was practically constant and equal to 100 X 10 4 ergs 

 for pressures between 2 and 4 mm. This is equivalent to an 

 average ionic energy of 35 volts (Table V.), assuming that 

 the ions do not ionize at the anode surface. It is easy to 

 show that this energy cannot be acquired by the ion in its 

 mean free path. The anode fall is about 20 volts, and this 

 fall occurs quite close to the anode, so close that a sound 



