305 



between about 130 deg. and 150 deg. This proved to be the 

 case; the improvement was considerable. Again with the new 

 arrangement the current with no radiator in position became 

 relatively far smaller. For example, when the radiator was 

 of Al, 4 mm. thick, and the absorbing screen D,D of tinfoil 

 (two thin sheets), the currents with and without the radiator 

 at B in fig. i. caused deflections of 86 and 26 mm. in ten sec- 

 onds respectively ; the currents with and without the radia- 

 tor at B in fig. ii. were 220 and 35 respectively. There could 

 be very little error, therefore, in taking the inci- 

 dence and emergence radiations as 60 and 185 respectively; 

 and the want of symmetry is beyond doubt. 



It should be observed that the emergence radiation can 

 never be shown to an unfair advantage in these experiments, 

 and is often at a disadvantage, for the radiator, when 

 placed as in fig. ii., cuts down the very primary rays to 

 which the secondary radiation is due. It is not difiicult to 

 show that if the thickness of the radiator is so adjusted as 

 to give the maximum emergence current (it can of course 

 be too thick or too thin), then the ratio of this maximum 

 to the maximum incidence current (which can be obtained 

 simply by making the radiator thick enough) is only 2/e of 

 the true ratio of emergence to incidence ; provided that the 

 secondary rays are as penetrating as the primary, and that 

 we are considering homogeneous radiations. But if, other 

 conditions being the same, the secondary rays are less pene- 

 trating than the primary, then the ratio as found is more 

 nearly correct, and is very nearly so when the secondary rays 

 are much less penetrating than the primary, as, for example, 

 when we are considering secondary cathode rays due to X- 

 or y rays. 



We have made a large number of measurements by the 

 method described above, using the following metal sheets as 

 radiators: — Pt, weight per square cm., *0150 gr. ; Sn, '0096 

 gr. ; Cu, -0083 gr. ; Fe, 0077 gr. ; Al, 105 gr. : celluloid, 

 '20 gr. As screens we have used various thicknesses of Sn, 

 Cu, and Al. 



The proportion of emergence to incidence radiation dif- 

 fers considerably for the different radiators, but is 

 much the same for different screens or different thicknesses 

 of screen, except that the proportion tends to increase slightly 

 as the screen is made thicker : and the tendency is most pro- 

 nounced in the case of those metals which give out a quan- 

 tity of soft secondary radiation. For example, Fe and Cu 

 show little difference between incidence and emergence radi- 

 ations until the screen is so thick that only a small fraction 

 of either of the radiations can pass through. The results 



