the Electrical Conduction produced by it. 

 Effect of Pressure. 



159 



Some current electromotive-force curves for small voltages 

 have been obtained at different pressures. Examples of the 

 results are shown in fig. 15. which gives the relation between 

 the current and the electromotive force at pressures of 760, 

 380, 190, and 95 mm. of mercury. 



These results were obtained with a different apparatus and 

 by a different method to that given in fig. 12. Two parallel 

 insulated metal plates, about 3 cm. apart, one of which was 

 covered with uranium oxide, were placed inside an air-tight 

 vessel. One plate was connected to earth and the other to 

 the electrometer. The plate connected to the electrometer 

 was then charged up to a potential of 10 volts. On account 

 of the presence of the uranium oxide the charge slowly leaked 

 away, and the rate of movement of the electrometer-needle 

 measured the current corresponding to different values of the 

 electromotive force. 



Fig. 15. 



























f/ 











* 





















/ 



Air 



190 MM 







k 



£■ 







1 



Hydroc 



EN< 760 



MM. — 











* / 



£ 







Air 9 



f MM. 





















V 





































Volt 



?. 









The method did not admit of the accuracy of that pre- 

 viously employed (see fig. 12). The rate of leak for small 

 fractions of a volt could not be determined, so that in the 

 curves fig. 15 it is assumed that the current was zero when 

 the electromotive force was zero. This is probably not quite 



