704 



Lord Blytliswood and Mr. Allen on 



the whole apparatus was placed in a large box lined with lead. 

 A \oncf wooden lever was arrancred so that contact between 



Fi' 



the insulated quadrant and '^ earth " might be made or 

 broken without the observer approaching the apparatus or 

 electrometer. 



In the first experiments made with this apparatus the 

 radium was protected by a loosely fitting cover of thin mica. 

 This allowed the "emanation"''' from the radium to enter the 

 testing-vessel, so that a potential-difference continued to show 

 itself after the radium was removed. In order to do away with 

 any uncertainty in the measurements arising from the presence 

 of the emanation with the accompanying excited activity, 

 the box containing the radium was hermetically sealed. The 

 cover was made of aluminium-foil, through which the radiation 

 [(3 and 7 rays) could penetrate. In this way a constant source 

 of radiation was secured. 



When making an observation a plate of the metal to be 

 tested was placed upon the lower electrode. Two readings 

 of the electrometer were taken, the first giving the potential 

 of the upper electrode when the lower was put to earth, 

 the second that of the lower electrode when the upper w^as 

 put to earth. The second reading was always about twice 

 as great as the first, the sign of the deflexion being, of 

 course, reversed. The smaller reading in the first case is due 

 to the fact that the testing- vessel (which was alw^ays con- 

 nected to earth) being of the same material as the electrode 

 receives part of the ionization current. 



