Mr. Norman Campbell on Delta Rays. 277 



on radiation of different types have shown that in general the 

 secondary radiation emitted by a body is in no way dependent 

 on its physical condition, or on the relation between the 

 atoms of which the body is composed ; it depends solely on 

 the nature o£ those atoms. Accordingly, analogy would 

 suggest that delta rays must be emitted also by gases under 

 the influence of alpha rays. It is apparently admitted* that 

 the process which appears as the emission of delta rays when 

 alpha rays fall on a solid is the same as that which appears 

 as ionization when the alpha rays pass through a gas. The 

 experimental difference between the two phenomena arises 

 from the facts that, in the case of the gas, the electrons are 

 emitted throughout a finite volume and not only at a surface, 

 that the electrons liberated combine with neutral atoms to 

 form ions, and that the positively charged remainders of the 

 atoms from which the electrons are emitted can move under 

 the action of an electric held. Jt was therefore suggested 

 to me by Prof. Bragg that the study of the delta rays has 

 been neglected unduly, and that the further investigation 

 of them might throw light upon the extremely important 

 problem of the mechanism of ionization. The work that has 

 been done hitherto! has been confined to a few measurements 

 of the speed of the electrons in special cases ; it appears 

 desirable to inquire by what circumstances that speed and 

 also the number of the electrons emitted are determined. 



2. In order to make a complete determination of the 

 velocity of the rays measurements must be made upon the 

 effect of both an electric and a magnetic field upon their 

 motion. Such measurements will give the value' ot both the 

 velocity and the ratio e/m. However, since in the present 

 investigation the value of e/m may be regarded as known, it 

 is sufficient to study the action of the electric field only. 

 The study of the action of a magnetic field according to the 

 method of J. J. Thomson J yields definite results only when 

 the rays are homogeneous, and is attended by the considerable 

 experimental difficulty of obtaining a perfectly homogeneous 

 magnetic field over a considerable area. Accordingly, a 

 magnetic field was employed only when it was desired to 

 prevent any of the rays leaving the surfaces from which 

 they were emitted ; no investigation of the effects of fields 

 less than that required for this purpose has been made. 



* Cf. for example. Kleeman, Roy. Soc. Proc. A. Ixxxiii. p. 195 (1910). 



t J. J. Thomson, Proc. Camb. Phil. Soc. xiii. p. 49 (1104) ; E. Ruther- 

 ford, Phil. Mag. x. p. 193 (1905 j ; Ewers, Phi/s. Zeit. vii. p. 148 (1900) ; 

 Log-emau, Roy. ;5oe. Proc. A. Ixxviii. p. 212 (1900). 



t J. J. Thomson, ' Conduction oi' Electricity through Gases/ p. G7. 



