An electron moving in a circular orbit is a powerful radiator of energy, since it is constantly accelerated towards the centre. An electron moving in an orbit of radius equal to the radius of an atom (about 10-8 cms.) would lose most of its kinetic energy of motion in a small fraction of a second, even though its velocity was originally nearly equal to the velocity of light. If, however, a number of electrons are arranged at equal angular intervals on the circumference of a circle and move with constant velocity round the ring, the radiation of energy is much less than for a single electron, and rapidly diminishes with an increase in the number of electrons round the ring. This result, obtained by J. J. Thomson, will be discussed in more detail later when the stability of systems composed of rotating electrons is under consideration.
Since the radiation of energy is proportional to the square of the acceleration, the proportion of the total energy radiated depends upon the suddenness with which an electron is started or stopped. Now some of the cathode ray particles are stopped abruptly when they impinge on the metal cathode, and, in consequence, give up a fraction of their kinetic energy in the form of electromagnetic radiation. Stokes and Weichert suggested that this radiation constituted the X rays, which are known to have their origin at the surface on which the cathode rays impinge. The mathematical theory has been worked out by J. J. Thomson[[97]]. If the motion of an electron is suddenly arrested, a thin spherical pulse in which the magnetic and electric forces are very intense travels out from the point of impact with the velocity of light. The more suddenly the electron is stopped, the thinner and more intense is the pulse. On this view the X rays are not corpuscular like the cathode rays, which produce them, but consist of transverse disturbances in the ether, akin in some respects to light waves of short wave-length. The rays are thus made up of a number of pulses, which are non-periodic in character, and which follow one another at irregular intervals.
On this theory of the nature of the X rays, the absence of direct deflection, refraction, or polarization is to be expected, if the thickness of the pulse is small compared with the diameter of an atom. It also explains the non-deflection of the path of the rays by a magnetic or electric field. The intensity of the electric and magnetic force in the pulse is so great that it is able to cause a removal of an electron from some of the atoms of the gas, over which the pulse passes, and thus causes the ionization observed.
The cathode rays produce X rays, and these in turn give rise to a secondary radiation whenever they impinge on a solid body. This secondary radiation is emitted equally in all directions, and consists partly of a radiation of the X ray type and also of electrons projected with considerable velocity. This secondary radiation gives rise to a tertiary radiation and so on.
Barkla[[98]] has shown that the secondary radiation emitted from a gas through which the rays pass consists in part of scattered X rays of about the same penetrating power as the primary rays as well as some easily absorbed rays.
Part of the cathode rays is diffusely reflected on striking the cathode. These scattered rays consist in part of electrons of the same speed as in the primary beam, but also include some others of much less velocity. The amount of diffuse reflection depends upon the nature of the cathode and the angle of incidence of the rays.
We shall see later ([chapter IV.]) that similar effects are produced when the rays from radio-active substances impinge upon solid bodies.
In this chapter an account of the ionization theory of gases has been given to the extent that is necessary for the interpretation of the measurements of radio-activity by the electric method. It would be out of place here to discuss the development of that theory in detail, to explain the passage of electricity through flames and vapours, the discharge of electricity from hot bodies, and the very complicated phenomena observed in the passage of electricity through a vacuum tube.
For further information on this important subject, the reader is referred to J. J. Thomson’s Conduction of Electricity through Gases, in which the whole subject is treated in a full and complete manner. A simple account of the effect of moving charges and the electronic theory of matter was given by the same author in the Silliman Lectures of Yale University and published under the title Electricity and Matter (Scribner, New York, 1904).