Noi.AN— Inni proilnced ill Air by Umlio-Aclu'c Bodies. 



.•?n 



Experimental Method. 



The apparatus (fig. 1) consists of a shallow rectangular box, measuring 

 internally 125 cm. in length, 3 1 •! cm. in width, and 10 cm. in depth. Insulated 

 metal plates are fixed flush with the top and bottom of the box. The lower 

 plate is 45 cm. long, and is connected to a potentiometer by which it can be 

 charged to any desired voltage. The upper plate, which is connected to an 

 electrometer, is 25 cm. long and 15 cm. wide, and is surrounded by a guard- 

 plate connected to earth. Thus the arrangement, as will be seen from the 

 diagram, is practically that of a parallel-plate condenser, the distance between 

 the plates being 10 cm. 'i'he ionization is produced at X, that is, directly 

 below the edge of the upper insulated plate. A detailed drawing of the 

 arrangement at ^r is given. It consists of a sort of trough of sheet lead, 

 1 cm. in width and l"2cm. in depth, which is sunk in the lower plate through 



Eorlh 



Electrometer 



ihz^ 



=tr 



ff ■ Radioactive moltci 



-DETAIL AT X- 



Fio. 1. 



a slot 1 cm. in width. This trough extends across almost the whole width 

 of the lower plate (approx. 28 cm.). A strip of lead 2-2 cm. in width is 

 supported above it at a distance of 1 cm. from the surface of the plate, 'i'he 

 radio-active matter is placed at the bottom of the I. rough. In these experi- 

 ments the source of radiation was a number of thin glass tubes containing 

 radium emanation. Thus a fairly intense local ionization is produced at the 

 region marked X, the intensely ionized strip extending across the width of 

 the lower plate, while the rest of the air-space is affected only by the 

 more penetrating radiation which has passed through a thickness of approxi- 

 mately 1'3 mm. of lead. A uniform current of air passes through the 

 apparatus in the direction indicated by the arrows. Tlie lower plate is 



[5*1 



