Page:Popular Science Monthly Volume 67.djvu/12

isolate the different kinds of rays and to determine their specific characters. The rays from the three most studied radio-elements, uranium, thorium and radium, can be separated into three distinct types, known as the $\alpha$, $\beta$ and $\gamma$ rays.

The nature of the $\alpha$ and $\beta$ rays has been deduced from observations of the deflection of the path of the rays by a magnetic and electric field. According to the electromagnetic theory, a radiation which is deflectable by a magnetic or electric field must consist of a flight of charged particles. If the amount of deflection of the rays from their path is measured when both a magnetic and an electric field of known strength are applied, the value of the velocity of the particles and the ratio $e/m$ of the charge $e$ carried by the particle to its apparent mass $m$, can be determined. From the direction of the deviation, the sign of the electric charge carried by the particle can be deduced.

Examined in this way, the $\beta$ rays have been shown to consist of negatively changed particles projected with a velocity approaching that of light. The experiments of Becquerel and Kaufmann have shown that the $\beta$ rays are identical with the cathode rays produced in a vacuum tube. This relationship has been established by showing that the value of $e/m$ is the same for the two kinds of rays. In both cases the value of $e/m$ has been found to be about 10⁷ electromagnetic units, while the corresponding value of $e/m$ for hydrogen atoms set free in the electrolysis of water is 10⁴. If the charge on the $\beta$ particle—or electron, as it has been termed—is the same as that carried by the hydrogen atom, this result shows that the apparent mass of the electron at slow speeds is about 1/1000 of that of the hydrogen atom. The $\beta$ particles from the radio-elements are expelled with a much greater speed than the cathode ray particles in a vacuum tube. The velocities of the $\beta$ particles from radium are not all the same, but vary between 10¹⁰ and $3\times 10^{10}$ cms. per second. The swifter particles move with a velocity of at least 95 per cent, of that of light. The emission by radium of electrons with high but different velocities has been utilized by Kaufmann to determine the variation of $e/m$ with speed. He found that the value of $e/m$ decreased with increase of velocity, showing that the apparent mass increased with the speed. By comparison of the experimental results with the mathematical theory of a moving charge, he deduced that the mass of the electrons was in all probability electromagnetic in origin, i. e., the apparent mass could be explained purely in terms of electricity in motion without the necessity of a material nucleus on which the charge was distributed. J. J. Thomson, Heaviside and others have shown that a moving charged sphere increases in apparent mass with the speed and that, for speeds small compared with the velocity of light, the increase of mass $m=2c^{2}/3a$ where $e$ is the