PRESENT PROBLEMS IN RADIOACTIVITY. 7 



charge carried by the body and a the radius of the conducting sphere 

 over which the electricity is distributed. Kaufmann deduced that 

 the value of e/m = 1.86 X10 7 for electrons of slow velocity. If the 

 mass of the electrons is electrical in origin, it is seen that a = 10 _ 13 

 cms., since the value of e = 3.4X10 10 electrostatic units. The re- 

 sults of various methods of determination agree in fixing the di- 

 ameter of an atom as about 10— 8 cms. The apparent diameter of 

 an electron is thus minute compared with that of the atom itself. 



The highest velocity of the radium electrons measured by Kaufmann 

 was, as we have seen, 95 per cent, of the velocity of light. The power 

 that electrons have of penetrating solid matter increases rapidly with the 

 velocity, and some of those expelled from radium are able to penetrate 

 through more than 3 mms. of lead. It is probable that a few of the 

 electrons from radium move with a velocity still greater than the highest 

 value observed by Kaufmann, and it is important to determine the value 

 of e/m and the velocity of such electrons. According to the mathe- 

 matical theory, the mass of the electron increases rapidly as the speed 

 of light is approached and should be infinitely great when the velocity 

 of light is reached. This leads to the conclusion that no charged 

 body can be made to move with a velocity greater than that of light. 

 This result is of great importance and requires further experimental 

 verification. A close study of the high speed electrons from radium 

 may throw further light on this question. 



Only a brief statement of our knowledge of electrons has been given 

 in this paper. A more complete and detailed account of both theory 

 and experiment will be given by my colleague, Dr. Langevin, in his 

 address on ' Physics of the Electron.' 



The a Rays. 



The (3 rays are readily deflected by a magnetic field, but a very 

 intense magnetic field is required to deflect appreciably the a rays. The 

 writer showed by the electric method that the rays of radium were 

 deflected both by a magnetic and electric field, and deduced the velocity 

 of projection of the particles and the ratio, e/m, of the charge to the 

 mass. The direction of deflection of the a rays is opposite in sense 

 to that of the /? rays. Since the /? rays carry a negative charge, the 

 a particles thus behave as if they carried a positive charge. The 

 magnetic deflection of these rays was confirmed by Becquerel and Des 

 Coudres, using the photographic method, while the latter, in addition, 

 showed their deflection in an electric field and deduced the value of the 

 velocity and e/m. The values obtained by Eutherforcl and Des Coudres 

 were in very good agreement, considering the difficulty of obtaining 

 a measurable deviation. 



