52 C. D. Cooksey — Secondary Cathode Jiays. 



is very small compared with the amount given out by metals 

 of high atomic weight, such as gold and silver. Unless the 

 ratio of emergence to incidence radiation is far different in 

 aluminium from what it is in gold and silver, which is unlikely, 

 the difference between m and n will be a quantity which we 

 may neglect, and we may write for a sufficient approximation 



e/i = a/b 



The determination of e/i, as above described, was always 

 repeated with the two plate holders reversed with respect to 

 the ends of A, which should give the same result if* both 

 plates were the same. The values obtained, however, in the 

 two cases differed very widely ; much more than could be 

 accounted for by the experimental errors. The only explana- 

 tion seemed to be that, owing to dirt on the surface or varia- 

 tions in the thickness of the metal foils, one plate was more 

 efficient in giving out cathode rays than the other. If this 

 were the case both the incidence and emergence raj's should 

 be effected in the same ratio, and the geometric mean of the 

 two determinations of e/i should give the same result. To test 

 this, different pairs of plates were made and also more foils 

 were added to the same plates, but no matter what the discord- 

 ance was between the two values of e/i for a given pair of 

 plates, the geometric mean of the value found when one plate 

 was on the front of A and the other on the back, and the 

 value when these positions were reversed, always gave the 

 same result within the limits of error of the experiment. 



This ratio, e/i, does not, however, give the true ratio of 

 emergence to incidence effect for the same intensity of exciting 

 rays owing to the fact that these were absorbed in the air of 

 the ionization chamber and to some extent in the layer of the 

 metal from which the cathode rays come. The ratio of the 

 number of emergence cathode rays coming from a layer of the 

 metal so thin that the exciting rays suffer no absorption in it 

 to the number of incidence cathode rays coming from the same 

 layer is the true value sought. Let this be designated by R. 

 Let /S be the coefficient of absorption of the cathode rays in 

 the metal from which they come, \ 1 the coefficient of the 

 exciting X-rays in the same metal, and \ their coefficient in 

 air. The number of emergence cathode rays produced in a 

 layer of thickness dx at a depth x of the metal by X-rays of 

 intensity I is equal to 



Kldx 



where K is some constant. If the intensity of the X-rays on 

 entering the metal is I , and the thickness of the plate t, the 

 number of the emergence cathode rays which get out of this 

 layer into the ionization chamber is equal to 



