Prof. Owens on Thorium Radiation. 



367 



where V is the potential-difference between the plates and i 

 the current, and takes into account the variation of electric 

 intensity between the plates. 



In the experiments to determine the saturation curves for 

 air ionized by thorium radiations, both the apparatus shown 

 in fig. 1 and that shown in fig. 8, to be described later, were 

 used. Very small voltages were not tried. 



With large voltages the conduction-current was independent 

 of the sign of the lower plate, corresponding in this regard to 

 uranium. 



Both the distances between the plates and the density of 

 the gas were varied. For a given air-pressure the voltage 

 required .to carry the saturation curve over the "knee" 

 was found to vary with the distance between the plates, 

 and when the plates were maintained at a constant distance 

 the voltage for the same purpose varied with the pressure of 

 the gas, being much more as the gas pressure increased. The 

 same general shape of curves was obtained when the surface 

 of the salt was covered with a thin layer of aluminium foil 

 as when the foil was removed. 



Table I. shows how the current varied with the voltage, 

 using a thick layer of thorium oxide covered with two layers 

 of thin aluminium foil, the plates being 5 centim. apart. The 

 results are expressed graphically in fig. 2 (p. 368). 



Table I. 



Thorium Oxide. Thick layer. 



Saturation Curve. 



Plates 5 cm. apart. Two layers of Aluminium foil 



over active material. 



Volts. 



Time of 100 div. in sec. 



Current. 



285 



11-3 



100-0 



185 



11-9 



95-0 



95 



130 



87-0 



54-5 



13-9 



81-0 



327 



14-9 



76-0 



21-8 



159 



710 



109 



186 



610 



55 



24-8 



45-0 



33 



350 



320 



2-2 



48-0 



24-0 



H 



720 



16-0 



