the Positive Ions emitted by Hot Platinum. 357 



A part of the irregularity in the decay of the positive 

 ionization may have been due to oxygen leaking into the 

 vacuum system. It may also have been due to negative 

 mixed with the positive ionization. As Professor Richardson 

 has already pointed out, the negative ionization increases 

 enormously with the temperature, and also does not begin to 

 appear in new platinum until a higher temperature is reached 

 than is necessary to obtain positive ions. Therefore the 

 observations for determining the kinetic energy of the posi- 

 tive ions were taken before the platinum strip had been 

 heated as much as two hours ; in fact, the observations were 

 taken just as soon as the hot platinum could be adjusted to 

 »ero potential and to a temperature satisfactorily constant ; 

 also the temperature was kept considerably lower than the 

 corresponding temperatures in connexion with the negative 

 ions. The temperature of the platinum strip ranged between 

 10G7 and 1293 degrees on the absolute scale, whereas the 

 corresponding temperatures used in obtaining the negative 

 ions were between 1473 and 1840 degrees absolute. 



The highest potential to which the upper plate was charged 

 by the positive ions was about 0*5 volt. For some time there 

 was some question as to whether there was not at that 

 potential an equilibrium point between the positive and the 

 negative ionization. As is well known *, the negative 

 ionization increases very rapidly with an increase in the 

 potential until the saturation-current is approached. In the 

 above case, there would be few or no negative ions reaching 

 the upper plate at zero potential : but as the plate charged 

 up, the number of negative ions would increase and the 

 number of positive ions would fall away, until the number of 

 negative ions became equal to the number of positive ; there 

 would then be no further change in the potential of the 

 upper plate. Obviously the theory that has been deduced 

 could not apply to the experimental results if such conditions 

 prevailed. That such conditions did not prevail was settled 

 conclusively by the following method : — Before and after 

 taking the observations, a positive potential of 2 volts was 

 placed on the upper plate, the hot platinum remaining at 

 zero potential, when it was found that there was no negative 

 leak. Clearly, then, there could be no negative ionization 

 when the upper plate was charged to 0*5 volt or less. The 

 fact that the pressure in the vacuum was frequently high, 

 tended to keep the negative ionization small. 



Otherwise than has just been stated, the procedure in the 

 ■manipulation was like that followed by Richardson and 



* Cf. O. W. Richardson, Phil. Trans. Series A, vol. xx. p. 520. 



