142 Prof. E. Rutherford on Uranium Radiation and 



The value of p\ can be deduced from the experi- 

 mental results, so that we have here an independent method 

 of determining the absorption of the radiation at different 

 pressures. 



The lower the pressure the more uniform is the ionization 

 between the plates, so that the saturation rate of leak at low 

 pressures is nearly proportional to the distance between the 

 plates. This is seen to be the case in fig. 8, where the curve 

 for a pressure of 187 mm. is approximately a straight line. 

 Similar results have been obtained for hydrogen and carbonic 

 acid. 



§ 13. Rate of Recombination of the Ions. 



Air that has been blown by the surface of a uranium com- 

 pound has the power of discharging both positive and negative 

 electrification. The following arrangement was used to find 

 the duration of the after-conductivity induced by uranium 

 radiation : — A sheet of thick paper was covered over with a 

 thin layer of gum-arabic, and then uranium oxide or uranium 

 potassium sulphate in the form of fine powder was sprinkled 

 over it. After this had dried the sheet of paper was formed 

 into a cylinder with the uranium layer inside. This was then 

 placed in a metal tube T (fig. 9) of 4 cm. diameter. A 



Fig. 9. 



Earth 



blast of air from a gasometer, after passing through a plug C 

 of cotton-wool to remove dust, passed through the cylinder T 

 and then down a long metal tube connected to earth. 



Insulated electrodes A and B were fixed in the metal tube. 

 The electrometer could be connected to either of the elec- 

 trodes A or B. In practice the quadrants of the electro- 

 meter were first connected together. The electrode A or B 

 and the electrometer were then charged up to a potential of 

 30 volts, and the quadrants then separated. 



When the uranium was removed there was no rate of leak 

 at either A or B when a rapid current of air was sent through 

 the tube. On replacing the uranium cylinder and sending a 

 current of air along the tube, the electrometer showed a 



