Page:Radio-activity.djvu/89

 and distance between the plates, the current does not increase beyond a certain value however much the activity of the material is increased.

48. Magnetic field produced by an ion in motion. It will be shown later that the two most important kinds of rays emitted by radio-active substances consist of electrified particles, spontaneously projected with great velocity. The easily absorbed rays, known as [Greek: a] rays, are positively electrified atoms of matter; the penetrating rays, known as [Greek: b] rays, carry a negative charge, and have been found to be identical with the cathode rays produced by the electric discharge in a vacuum tube.

The methods adopted to determine the character of these rays are very similar to those first used by J. J. Thomson to show that the cathode rays consisted of a stream of negatively electrified particles projected with great velocity.

The proof that the cathode rays were corpuscular in character, and consisted of charged particles whose mass was very small compared with that of the hydrogen atom, marked an important epoch in physical science: for it not only opened up new and fertile fields of research, but also profoundly modified our previous conceptions of the constitution of matter.

A brief account will accordingly be given of the effects produced by a moving charged body, and also of some of the experimental methods which have been used to determine the mass and velocity of the particles of the cathode stream.

Consider an ion of radius α, carrying a charge of electricity e, and moving with a velocity u, small compared with the velocity of light. In consequence of the motion, a magnetic field is set up around the charged ion, which is carried with it. The charged ion in motion constitutes a current element of magnitude eu, and the magnetic field H at any point distant r from the sphere is given by

H = eu sin θ/r^2,