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Mr. A. J. Saxton on Impact Ionization by 



4 volts between the side electrodes Ei and E 2 , and putting 

 both G 2 and D to earth. Ei was maintained at a potential 

 of —4 volts, and E 2 connected to the electrometer. Since E 2 . 

 was positive with respect to the electrode E 1? it should collect 

 the negative ions produced by collision. It was found,, 

 however, that E 2 was always charged up positively by an 

 amount which increased with the accelerating P.D. of the 

 positive ions. The effect is probably due to the formation 

 of a positive space-charge in the ionizing chamber. This 

 posilive charge deflects the incoming positive ions to the 

 side electrodes. 



5. Description of Apparatus 2. 



To obtain more definite results, two improvements on the 

 old apparatus were desirable. The positive-ion current from 

 the filament itself must be eliminated. It was also desirable 

 to work at much lower accelerating voltages to detect the- 

 potential at which ionization sets in. A diagram of the 

 second apparatus is shown in fig. 4. The side electrodes 



Vvolts 



Fig. 4. 



Ace. P. D.i Ionization 



->K- 



Drawinq- out R D I 

 ~ G 



y 



Chamber 



E x and E 2 were omitted and the end electrode E increased 

 in. area. The tungsten filament F was placed in a side tube 

 to cut off the positive ions given off by the filament itself. 

 The filament was surrounded by a nickel-foil screen, which 

 rested on the top of the cylinder A. 



Electrons from F were accelerated by the- arcing poten- 

 tial V into the nickel cylinder A through the gauze opening, 

 and there produced positive ions in the hydrogen by collision. 

 A P.D. of x volts is applied between A and Gi to draw out 

 the positive ions, which, passing through the gauze Gi, are 

 then accelerated by a P.D. of y volts between Gi and G 2 - 

 A retarding P.D. for positive ions of z volts is applied in- 

 the ionization chamber between G 2 and E. 



