Oil Chayu'iDii t/n'Oiiif/i fori Ah.^ovpt/on. ^)'^^d 



li< activity is probably about 100 times that of uranium. The 

 divergence of the leaves biggins to be perce})tible about three 

 hours after complete disciiarge, and the full diveroenc3 is 

 attained in about twenty hours. For lecture purposes it would 

 be better to have a more active ])reparation, so that the ccm- 

 })lete cycle would be gone through in the course of an hour. 

 If the apparatus is sealed oti' from the pump it will be 

 available for exhibition at any time. But very complete 

 exhaustion is necessary. 



The a rays of radium, which are deflected by a magnet in 

 the opposite sense to the ^ rays, should carry a positive charge. 

 I have attempted to detect this by the same method. A brass 

 strip, carrying an aluminium leaf, was supported on a quartz 

 rod, and itself supported in turn a small dish, ou which a 

 radium [)reparation was spread. The whole was arranged in 

 a vessel connected to earth, and exhausted. No deflexion 

 was, how^ever, obtained. In order to test the insulation a 

 charge was given to the brass strip, causing the aluminium 

 leaf to diverge. But even at the best vacuum which the 

 Toepler-pump could produce, the charge was lost in an hour. 

 The ionization produced in gases by the a rays is so strong 

 that even at the lowest pressure of the mercurial pump it 

 prevents the detection of the charge carried by these rays. 



LXXY. Oil Charging through Ion Absorption and its Bearing 

 on the KartKs Permanent JSlegative Charge. By George 

 C. Simpson, B.Sc.^ 1851 Exhibition Scholar, Owens College'^. 



OF the numerous theories for the permanent negative 

 charge on the earth^s surface none has been so generally 

 accepted by geophysicists as that due to Elster and Geitel^ 

 which ascribes the phenomenon to a charging of the surface 

 through absorption of ions from the surrounding atmosphere. 

 The explanation given by H. Geitel in his Hamburg address 

 (" Ueber die Anwendung der Lehre von den Gasionen auf 

 die Erscheinungen der atmospharischen Electricitat,'''' pub- 

 lished by Vievveg & Sohn, Braunschweio-, 1901) was as 

 follows :— Zeleny has shown (Phil. Mag. xlvi. p. 120, 1898) 

 that when insulated conductors are in contact with Ront- 

 genized air they become negatively charged in consequence 

 of the greater mobility of the negative ions. This charging 

 has a limit which is reached when the field set up is strong- 

 enough to make the number of positive and negative ions which 

 reach the surface equal, by increasing the velocity towards the 



* Communicated by the Author. 



