during the Absorption of Electrons by Platinum. 189 



§ 7. Discontinuity at Higli Potentials. 



An interesting phenomenon was observed when the investi- 

 gation was pushed to higher potentials than those so far 

 recorded. It was found, for example, that with the platinum 

 grid which had been exposed to hydrogen, when potentials 

 higher than 33 volts were used to measure the effect a smaller 

 heating effect was obtained for a given thermionic current 

 than at somewhat lower voltages. When the matter was 

 examined in greater detail it was found that at a certain 

 voltage there was a sharp drop in the value of the heating 

 effect per unit current. In the experiment under discussion 

 this took place at 33 volts. From 36 to 50 volts the heating 

 effect per unit current appeared to be almost independent of 

 the voltage. The sudden drop in the heating effect was found 

 to be accompanied by a simultaneous discontinuity in the 

 current-E.M.F. curve. In figure 3 the heating effect and 

 the current-E.M.F. are plotted together. The points on the 

 current-E.M.F. curve are shown thus O, and the heating- 

 effect thus ©. We have not yet had time to examine this 

 interesting phenomenon in detail, but two possible explana- 

 tions suggest themselves. One is that for some reason or 

 another when the voltage exceeds the critical value the dis- 

 charge, or part of it, takes place to the part of the grid in 

 the immediate neighbourhood of the glass supports, and part 

 of the heating effect is conducted into the glass and does not 

 make itself felt in the grid. The second, which is more 

 interesting, is that the phenomenon is connected with the 

 reflexion of the electrons or the emission of secondary elec- 

 trons at the surface of the metal. In that case it seems quite 

 conceivable that there may be a sudden increase in the 

 amount of this effect at a certain potential : that the 

 secondary electrons thus emitted escape from the grid with 

 considerable velocities, manage to pass out of its sphere of 

 action, as it were, and drift into the other parts of the field ; 

 so that, for example, instead of entering the grid and ulti- 

 mately passing through the instrument which measures the 

 thermionic current they reach the positive terminal of the 

 osmium filaments. Some such view would account for the 

 simultaneous discontinuity in both the heating effect and 

 the thermionic current. In order to account for the fact that 

 at higher potentials the heating effect is independent of the 

 voltage, we should have to suppose that the kinetic energy 

 of the particles thus lost by the grid increased in a greater 

 ratio than that of the potential driving them. When the 

 investigation was pushed to still higher potentials it was 



