Further Experiments on Delta Rays. 527 



This result is in practical agreement with what had 

 previously been found to characterize the radiation at Toronto, 

 and this characteristic, taken in conjunction with the low 

 value found for " q " over the waters of Lake Ontario, is what 

 led the writer to conclude that the soil of the earth and not 

 its atmosphere nor any of the celestial bodies was the chief 

 source of the penetrating radiation. 



In conclusion, I wish to acknowledge my indebtedness to 

 my assistants Mr. P. Blackman and Miss A. T. Reed, for 

 making some of the reductions and for taking a number of 

 the readings at Toronto. 



Physical Laboratory, University of Toronto. 

 May 1,1912. 



L. Further Experiments on Delta Rays. 

 By Norman Campbell, Sc.D* 



1. TN three recent papers t the problem of the velocity of 

 JL the delta rays and its dependence on the velocit}' 

 of the exciting alpha ray and the material from which they 

 are emitted has been discussed. At the end of the last paper 

 it was pointed out that most of the phenomena which had 

 been observed could be reconciled with the view that the 

 delta rays possessed no finite initial velocity greater than a 

 small fraction of a volt and that this view, if it could be 

 accepted, would simplify greatly the theory of ionizaiion. 

 The general method of experiment had been to determine 

 the relation of the current flowing between two plates, 

 from which delta rays were being emitted, to the potential 

 difference between those plates, and there was some evi- 

 dence that this relation was determined as much by the 

 energy required to drag the slowest rays away from the 

 place where they originated as by the energy required to 

 prevent the fastest rays leaving that place. 



But all observers agree that delta rays can leave a p]ate 

 struck by alpha rays, even when that plate is at a positive 

 potential of some volts with regard to its surroundings. 

 The rays must, therefore, be leaving the plate with a finite 

 velocity, and if, as has been suggested, this velocity is not 

 conferred on them in the act of emission, some source of 

 energy competent to confer it after they have been emitted 

 must be found. When the suggestion was put forward, it 

 appeared that such a source of energy might be found in a 



* Communicated by the Author. 



t Phil. Mag. Aug. 1911, p. 276: Jan. 1912, p. 46 ; April 1912, p. 462. 



