﻿198 



Dr. J. N. Pring and Mr. A. Parker on the 



the abscissse, the pressures. In fi^. 3 are seen the results of 

 another series of experiments which were extended to much 



Fiff. 3. 



liOf' 1 1200° /Jl00° /S00° /600° tfOO* /t00° /300* 2.000" 



* TEMPERATURE 

 lower pressures, and where a higher degree of purity of the 

 carbon was obtained. In tills diagram the ordinates denote 

 the logarithm of the ionization currents (in amperes) for one 

 square cm. of cathode surface, and the abscissse, the tempe- 

 ratures. The results obtained by Richardson are shown in 

 this diagram at A. In the present work it was found that 

 the minimum of ionization is not obtained at the pressures 

 used by Richardson, but that the values fall continuously 

 with diminution of pressure and with continued use of the 

 same carbon as it becomes progressively purer. Beginning 

 with h'gher pressures and unpurified carbon, it was found 

 that the maximum ionization current is obtained at about 0*1 

 mm. pressure and that the values are about one-tenth those 

 given by Richardson as the minimum for the same tempera- 

 ture. The values at O'OOl mm. were about ^ 6 those at 

 0*1 mm. In the experiments at higher pressures with 

 impure carbon, shown in curves B to E, at temperatures 

 between 1250 c and 1820°, if the values of log I — \ log 6 are 



plotted against -» 10 4 , a straight line is obtained ; but at 



higher temperatures, which were taken up to 2050°, and 

 lower pressures, the ionization increased with the tempera- 

 ture far less than is required by Richardson's formula, and 



