466 Mr. J. J. Lonsdale on the Ionization 



the ionization current could be attributed to the electrification 

 of the mercury. 



To ensure that no variation in the ionization was due to 

 the variations in the state of the mercury, different samples 

 were tried and the currents taken. If the mercury were 

 purified by bubbling air through for a considerable time 

 before using, the results showed no variation. 



The effect of changing the surface on which the mercury 

 splashed was also investigated. Previous investigators have 

 stated that the material of the plate has no effect, at least if 

 the plate is wetted by the liquid. The electrometer having 

 a sensitiveness of about 800 mm. scale-divisions for a volt, 

 the mercury splashing on iron gave a current of 120 mm. 

 in 20 seconds due to positive ions. If the splash-plate were 

 re-cleaned and polished the variation was very slight. The 

 negative ionization was very much less. A voltage of about 

 200 volts was put on the second electrode in both cases. 

 A platinum splash-plate gave under similar conditions a 

 positive current of about 30 mm. in 20 seconds. The 

 negative current was scarcely measurable. When mercury 

 was dropped on mercury the positive current was very 

 small, often nothing ; the negative current was larger. Glass 

 was tried as presenting points of interest. Very variable 

 currents were observed, as might have been expected, owing 

 to the field produced near the source by the electrification 

 of the glass. If the glass plate was used once and then 

 heated to discharge it, the results were in moderate agree- 

 ment and gave a positive current of about 14 mm. in 20 seconds. 



With the apparatus used it has been shown that the 

 mobility of the ions is given by * 



(b 2 -a 2 ), b 



where b is the radius of the outer tube; 



a is the radius of the inner electrode ; 

 Y is the voltage on the outer tube ; 

 t the time taken by the blast to pass along the length 



of the electrode ; 

 p the ratio of the saturation currents on the second 

 electrode. (1) when field V is on D ; (2) when 

 there is no potential difference between D and the 

 tube. 

 A first examination of the positive ions showed that the 

 current gradually rose as the voltage increased and then 

 remained constant between voltages of 85 and 150 volts. 

 * J. J. Thomson, ' Conduction of Electricity through Gases/ p. 59. 



