e 



MR. O. W. RICHARDSON ON THE lONISATIo.V 



It will be seen that the maximum current is obtained with the potential at the 

 middle point of the filament equal to +175 volts. Since there was a fall of potential 

 of 3'3 volts along the filament due to the heating current, the more negative end 

 would only be at a potential of +0'1 volt when saturation occurred; so that it is 

 evident that the only competition the electrode experiences in collecting the ions is 

 that due to the filament itself. 



The mean potentials in cases where low voltages were used were determined by 

 connecting a Weston voltmeter, one terminal of which was earthed, to each end of 

 the filament in turn. The gradual decrease in the current as the potential rises 

 from 175 to 38, which is very evident from the above numbers, will be dealt with 

 later. 



These current E.M.F. curves which show saturation at about 2 volts were only 

 obtained at low temperatures. At higher temperatures the necessary voltage went 

 up to about 40, as is shown by the following table : 



Temperature = 1180C. 



Pressure = 89 millims. 



The greater difficulty experienced in reaching saturation at high temperatures may 

 possibly be due to the relatively greater magnitude of the negative ionisation which 

 would make recombination a factor to be reckoned with. 



The experiments show that a potential of +80 volts is sufficient to saturate the 

 current from a platinum wire at all the temperatures used. This is of great 

 importance in the sequel, where the leak with 80 volts is used to measure the total 

 number of ions produced by a platinum wire under various conditions. 



When the pressure of oxygen in the apparatus was of the order of one millimeter, 

 the current ceased to be independent of the voltage at high voltages. A careful 

 series of measurements was therefore made of the way in which the current varied 

 with the voltage at different pressures. The results are given in the next table and 

 are also exhibited graphically in fig. 2. The measurements at the lowest pressure 

 (O'OG millim.) were made at a temperature of 822 C. ; for all the other pressures the 

 temperature was 808 C. To eliminate time changes, the leak with + 80 volts was 

 taken as a standard and was measured both before and after each observation with 

 another voltage. This precaution was not really necessary, as the steady state had 

 been reached before the measurements commenced, but it served as a useful check. 

 On account of the method of taking the observations, and also because they do not all 

 refer to the same temperature, the absolute values of the leaks have not been given, 

 but for each pressure they are referred to the current with +80 volts as a standard 



