Page:Popular Science Monthly Volume 67.djvu/13

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 $e/m=1.86\times 10^{7}$ 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 $e=3.4\times 10^{10}$ electrostatic units. The results of various methods of determination agree in fixing the diameter 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 mathematical 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 $\beta$ rays are readily deflected by a magnetic field, but a very intense magnetic field is required to deflect appreciably the $\alpha$ 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 $\beta$ rays. Since the $\beta$ rays carry a negative charge, the $\alpha$ 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 Rutherford and Des Coudres were in very good agreement, considering the difficulty of obtaining a measurable deviation.