Experiments on Delta Rays. 533 



a surface than fall upon it. If a fast delta ray which can 

 travel against a potential difference of 10 volts excites at D 

 more than one secondary ray, then there will obviously be 

 a positive current to D which will be stopped when the delta 

 rays are prevented from reaching D by the application of a 

 magnetic field. And since v. Baeyer found that soot shows 

 practically no reflexion, the difference which has been noted 

 between soot and metals is immediately clear. 



In a similar manner it may be possible to explain a 

 curious phenomenon which, though it was suspected in 

 the previous experiments, was not mentioned for lack of 

 certainty. It now appears that it has also been observed 

 by Pound (loc. cit.) and there can be no further question 

 as to its reality. In such measurements as were described 

 in the first two papers, especially when the electrodes were 

 covered with aluminium, it was found that an increase in 

 the potential difference between the electrodes bevond the 

 16 volts necessary to attain saturation to 40 or even 

 100 volts produced a small but noticeable decrease in 

 the current *. The only explanation I can offer of this 

 decrease is that a delta ray from the negative electrode 

 sometimes attains a sufficient speed in travelling through 

 the 16 volts to enable it to liberate a secondary ray which 

 can travel back to the negative electrode against the field: 

 such an action presents no difficulty on the score of energy, for 

 the rays start from the negative electrode with a finite speed. 

 The number of such secondary rays liberated would increase 

 with the potential difference between the electrodes so long 

 as the power of exciting secondary rays increased with 

 increase in the velocity; v. Baeyer has shown that a stage 

 is reached later when the power of exciting secondary rays 

 decreases with increase in velocity ; but I have never been 

 able to observe in the cases mentioned a second increase of 

 the current with the potential difference. 



9. If the explanation offered is correct, it is clear that the 

 form of the curves will be materially influenced by the 

 reflexion of the delta rays at the electrode D, unless this 

 electrode is covered with soot. In all other cases the quantity 

 i v Ji will not represent accurately the proportion of the rays 

 which have a velocity greater than V, for the number retained 

 by D will depend, not only on the number of rays striking it, 

 but also on the number reflected from it. But unless A is 

 also covered with soot, we introduce another source of error 

 in covering D with soot rather than with the same material 



* See, e.g,, Table 2, column 1 (ppge 534). 

 Phil. Mag. S. 6. Vol. U. No. 142. Oct. 1 ( J12, 2 N 



