[kobertson] insulated METALLIC COxHDUCTORS 



TABLE XVIIL 



131 



One should have expected that as the radium was removed the values 

 given in column II of Table XVIII would have approximated to those 

 obtained without the radium and recorded in column I, b'ut their failure 

 to do so can probably be traced to the difficult}' experienced in obtaining 

 satisfactory readings when the radium was placed at the longer distances 

 or entirely removed. Under these circumstances the rate of deflection 

 was exceedingly slow and, consequently, the readings, owing to the 

 comparatively small capacity of the plate system, were subject to large 

 errors arising from disturbances to the measuring system which with 

 the radium nearer were negligible. 



In analyzing the observations made on the Yolta effect in the 

 plate experiments it is difficult to account for all the results. It is 

 evident that at least two influences were present, namely, (1) the con- 

 ductivity of the air between the plates, and (2) secondary radiation, 

 and it is clear that each contributed, with the différent combinations, 

 to the magnitude and sign of the charge acquired. 



That ■conductivity is an important factor in determining the limit- 

 ing charge given to the free plate was shewn by simply blowing filtered 

 air between the plates. It was found that in all cases, where the upper 

 plate was positive to the lower, positive values were decreased by blow- 

 ing. On the other hand, by using a carbon plate above, so as to make 

 the copper plate below acquire a negative charge through the Volta effect, 

 the negative deflection was also lessened by the same means. 



It has been she'WTi by Prof. MacKenzie ^ that the secondary radia- 

 tion from the back of metallic plates, upon which the rays from radium 

 are allowed to fall, is of considerable importance. Negative particles 

 will, under these circumstances, come to the lower plate from the upper, 

 while there will also be an emission of negative particles from the 

 lower one itself. The nature of the charge acquired by the lower platx? 



^ Phil. Mag., July, 1907. 



