58 



MR. O. W. RICHAEDSON ON THE IONISATION 



impossible to saturate it. This is shown by the following numbers for the positive 

 current under different voltages at 809 C. : 



A number of experiments were made to find the cause of this anomalous behaviour 

 at low temperature, but no satisfactory conclusion was arrived at. Fortunately, this 

 does not matter much as far as the present investigation is concerned, for the 

 experiments described below were all carried out at much higher temperatures, when 

 saturation was very nearly attained with 80 volts. 



Experiments on the variation of the positive ionisation with the rate at which 

 hydrogen was diffusing through the walls of the tube were made at 1200 C. 

 approximately. The rate of diffusion was varied by varying the pressure of the 

 hydrogen inside the apparatus, since the quantity diffusing in a given time has been 

 shown* to vary very nearly as the square root of the pressure inside. The way in 

 which the saturation current varied with the pressure P of the hydrogen inside the 

 tube is shown by the following numbers : 



The numbers in the last column were calculated by assuming that the current was 

 equal to a + ftP*, a and b being constants. The agreement of the results shows that 

 the leak consists of two parts, one independent of, and the other proportional to, the 

 square root of the pressure of the hydrogen inside the tube. The effect of the 

 hydrogen diffusing out of the platinum, therefore, is to produce an additional number 

 of positive ions proportional to the amount of hydrogen diffusing out. 



These results tend to indicate that the hydrogen inside the metal, which is known 

 from other considerations to be in the atomic state, is positively charged. Only a 

 small fraction (about 10~ 7 ) of the hydrogen comes out in the ionic form, but on 



* RICHARDSON, NICOL, and PARNELL, loc. cit. 



