LM'LENNAN] ON ELECTRIC CHARGES 89 



this would explain the acquisition of an initial negative charge when the 

 earth connection to the free quadrants was broken. It is probable, how- 

 ever, that the difference between 20 and 13.3 did not represent the exact 

 magnitude of the volta effect, for in this case the radiations present would 

 include secondary rays from the walls of the brass outside cylinder as 

 well as others from the walls of the inner aluminium tube. The reading 

 13.3 would then represent the equilibrium potential acquired by the free 

 system through the agency of (1) the volta effect, (3) the conduction 

 current, (3) the secondary rays from the aluminium tube, (-i) the se- 

 condary rays from the brass outside vessel and also (5) any tertiary rays 

 from the two opposing surfaces which might be present. The initial in- 

 crease in the negative charge acquired by the aluminium tube in the 

 range of pressures extending to 80 mm. was probably due to modifica- 

 tions produced by the lowering of the pressure in the amount of second- 

 ary rays coming to the aluminium tube from the walls of the brass out- 

 side vessel. The charging action of these would initially mask the charg- 

 ing action of the various excited secondary rays issuing from the surface 

 of the aluminium tube. But ultimately, as the pressure was lowered, a 

 point would be reached when these secondary rays from the outside vessel 

 would exert their maximum effect. For lower pressures the conduction 

 current through the gas would become less and less. This would bring 

 the secondary rays from the aluminium tube more into evidence and so 

 account for the rapid increase in the positive charge acquired by the free 

 system, which the curve shows took place at the lowest pressures. From 

 this experiment it will be seen that the charging action of the secondary 

 rays produced in aluminium by the gamma rays from radium was 

 brought into evidence at a very much higher pressure, 80 mm., than a 

 similar action by the rays from polonium. 



TV. On the Charges Acquired by Uranium Salts at Low 



Pressures. 



In this experiment the form of the vessel used in making the ex- 

 amination is shown in Fig. III. A flat tray of brass BB was supported 

 by an ebonite plug on the brass plate CC shown in the figure. A shallow 

 brass cover FF was placed over the tray BB and rested on the lower plate. 

 Over the whole a second cover DB was placed which fitted snugly in to 

 a groove provided in the lower plate CC. In making the experiment the 

 salt to be examined was placed on the tray BB, the covers were then 

 placed in position, and finally all the joints made air-tight with sealing 

 wax. The salt used in making tlie experiment was a sample of uranium 

 nitrate. The measuring instrument used was again a quadrant electro- 

 meter, but in this case it possessed a sensibility of 600 divisions per volt. 



Sec. III.. 19(i9. 6. 



