Radioactivity of Metals oenerallt/. 347 



reach it sliglitly inereastnl with the use of cylinders of smaller 

 diameter. 



The experiments were also varied by placing the lead 

 cylinders in a tank filled with water, the surrounding layer 

 of water having a thickness of 13 cms. Under these conditions 

 the limiting potential was found to be unchanged. 



It was also observed that this metal effect was produced 

 when Rontgeu and Becquerel rays were allowed to traverse 

 the cylinders. "With these radiations, however, the limiting 

 negative potentials were reached in the course of a few 

 seconds and varied but little from the values obtained for the 

 different metals under ordinary conditions. 



A small quantity of radium chloride of activity 1000 con- 

 fined in a glass phial was used as the source of Becquerel 

 rays, and was placed, in the experiment, on a support close 

 to the outside of the cylinder B. The effect was also pro- 

 duced with a quantity of uranium oxide placed near the 

 cylinder, but a much longer time was required than with the 

 radium to reach the steady state. 



The followino- are the values of the limitino; neorative 

 potentials obtained under the chfferent conditions. 



INeojatiTe Potential in ^lilliTolrs. 



Meial. 



[Natural 

 Eacliation. 



Aluminium IT'J 



Zinc 160 



Lead 216 



E<)ntgen Eays. Becquerel Eays. 



179 179 



176 180 



175 201 



I 



Tin 95 98 100 



Copper 73 72 69 



Volt a Efect. 



It is CAndent that with cylinders of different metals the 

 arrangement shown in fig. 1 afforded a means of exhibiting 

 the Yolta effect, and of approximating to a measure of the 

 contact differences of potential for a series of metals. 



ATith the rod D in position a difference of potential would 

 exist between the cylinders A and B. Its withdrawal would 

 be followed by a gradual equalization of these potentials which 

 would result in a charge being communicated to the con- 

 necting wire and the free quadrants. This again would set 

 up a current to the surrounding tube C, and finally the free 



