288 Cooksey— Corpuscular Rays produced in 



bored in them could be hung in front of " W " to control the 

 amount of rays entering " A." A shutter, " d," placed over 

 " W " and controlled by a screw " e," gave a very tine adjust- 

 ment to the size of this opening. 



In use the lead box, chamber " A," and electroscope case 

 were all connected to earth ; the chamber " B " was kept at a 

 constant potential of about 400 volts, and the gold leaf, rings 

 " c " and " c'," and connections were brought initially to about 

 200 volts. This made a difference of potential between " A " 

 and "c" of 200 volts, and between "B" and "c"' of minus 

 200 volts. The fall of the gold leaf due to the ionization in 

 the chamber "A" could be read with a microscope having a 

 micrometer eyepiece. If the Rontgen rays were turned on, and 

 the size of the opening, "W," properly adjusted so that there 

 should be the same amount of ionization in " B " as in " A," 

 the flow of electricity from " c " to " A " would be just equal 

 and opposite to the flow from "B" to "c' 5 " and there would 

 be no motion of the gold leaf. In this way the ionization due 

 to the Rontgen rays and any natural leak could be eliminated. 



In order to test a given metal, a disk of 'the metal was made 

 to fit over the rear end of the chamber, "A." This disk was 

 then clamped over the end with a thin disk of aluminium 

 interposed between it and the primary rays. (Aluminium was 

 used because it gives off hardly any of the corpuscular second- 

 ary rays, and there do not enough of the secondary Rontgen 

 rays come off from a thin sheet to produce any appreciable 

 ionization.) The Rontgen tube was then excited, and the 

 shutter, " d," adjusted till there was no motion of the gold 

 leaf. The aluminium screen was then removed, and the time 

 taken for the gold leaf to fall over a given number of scale 

 divisions measured. The fall of the gold leaf was then due to 

 the ionization produced by the secondary rays alone from the 

 metal. 



Preliminary experiments showed that an appreciable amount 

 of the ionization in the chamber, " A," was due to secondary 

 rays of the Rontgen type, and it was therefore necessary to 

 find what proportion of the ionization was due to corpuscular 

 secondary rays, and what due to secondary Rontgen rays. For 

 this end the absorption of the secondary rays in thin sheets 

 of aluminium leaf was measured: The primary rays were 

 first eliminated by balancing the apparatus with a sheet of 

 aluminium over the metal about l ram thick, sufficient to stop 

 all the secondary rays from the metal, but not thick enough 

 to give off any appreciable secondary rays itself that would be 

 absorbed in the ionization chamber. The ionization was then 

 measured which was due to the secondary rays from the 

 bare metal and to those which got through successive sheets of 



