JRiuIiiini Uad'hiflon dncl Contact Kleclricitij. 



Table I. 



Deflexion tor Clark Cell + U5, - U')0. 



'03 



Metal on Lower Plate. 



Potential of Brass Plate. 



Deflexion. 



Volts. 



Aluiuiiiium 



+40 

 + 35 

 +32 



- 2 



— 7 

 -13 

 -•.:;0 

 -23 



+•40 

 + •35 

 + •32 

 -•02 I 

 -06 

 -12 1 

 -•18 



-•21 i 



1 

 i 





Lead 



Copner 



Silver (five-shilling piece) ...... 



Gold 



Mercury (in a watob -glass) 





As was to be expected, it was found that the deflexion 

 observed was dependent on the state o£ the metal surface. 

 Thus an aluminium plate showed a deflexion of 10 after 

 polishing with the " buff," which would leave traces of grease 

 on the surface, but a deflexion of 40 after it was cleaned with 

 fine emery-paper. 



Potential-Differences obtained loith a Lead Plate. 



Several observations made during our preliminary expe- 

 riments showed the importance of limiting the effects to the 

 two metals it was proposed to examine. As it was necessarv 

 to inclose the apparatus and the radium salt in a lead case 

 to prevent interference from external sources, it was decided 

 to make a new apparatus to consist wholly of lead. In this 

 way all the subsequent determinations would measure the 

 potential- difference between lead and any one metal. This 

 had a further advantage from the fact that lead lies at one 

 end of the voltaic series, so that by employing it as a standard 

 larger deflexions of the electrometer could be obtained. The 

 new apparatus is shown in fig. 2 (p. 704). 



The lead plates were circular disks with faces 5 cms. in 

 diameter. They were put in connexion with the electrometer- 

 quadrants by lead rods passing through glass tubes. The 

 distance between them was 2 cms. The radium was con- 

 tained in a small lead box which could be slipped into a tube 

 at the side of the case. The sample used was obtained through 

 Isenthal and Co. It was considerably more active than that 

 used in the preliminary experiments. For further security 



3 A 2 



