RADIOACTIVITY VISUALISED 1 



By C. T. R. WILSON, M.A., F.R.S. 



The phenomena of radioactivity are known to be due to the 

 ejection from the atoms of the radioactive elements of two 

 kinds of particles which travel with enormous velocities : 

 (i) the alpha-particle, which is a positively charged helium 

 atom having a mass four times that of the hydrogen atom ; 

 (2) the beta-particle, which carries a negative charge only 

 half as large as the positive charge of the alpha-particle and 

 has a mass less than the 1700th part of the hydrogen atom. 



The velocity of the fastest beta-particles approaches very 

 nearly to that of light, that of the alpha-particles being con- 

 siderably less but still exceeding 10,000 miles a second. 



By the action of Rontgen and other radiations, we can 

 cause electrons or corpuscles which are identical with the 

 beta-particles to be expelled from the atoms of any element 

 with velocities comparable with those with which the alpha- 

 particles are ejected from radium. 



The methods which have been used hitherto in the study 

 of the paths of these projectiles and of the effects produced 

 by them in their flight have been somewhat indirect. The 

 actual paths of individual particles have not been observed ; 

 it has been necessary to investigate the combined effects of a 

 large number of particles. 



It is true it has been found possible by two different methods 

 to detect effects arising from the action of a single alpha-particle. 

 Thus Rutherford introduced a method in which effects due to 

 the ions set free along the path of a single alpha-particle could 

 be detected by an electrometer ; again in the Crookes spin- 

 tharoscope each alpha-particle causes a starlike point of light 

 to flash forth momentarily where it strikes the prepared screen. 

 But it has not been found possible by such methods to detect 

 effects arising from a single beta-particle. 



1 A lecture delivered at the Royal Institution on the evening of Friday, 

 February 28, 1913. 



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