The energy given to the atom is now measured in tens of thous ands of volts or even hundreds of thousands of volts, and not in tens of volts as in the case of light. It is to be expected that different methods of investigation must be used to meet the changed conditions, but besides this there is a shift of the point of interest. The amount of energy transferred to the electron is so large that the energy lost in getting free of the body as a whole is negligible in comparison, but on the other hand the large energy of the quantum enables electrons to be removed from even the innermost orbits of the atom. Radiation of one fre quency can thus give rise to a series of electron groups of differ ent energies, according to whether they are ejected from the outer or inner orbits of the atoms. The electrons from the innermost orbits will have used up the greater portion of their hv, and will therefore emerge with the lowest velocities, whereas those from the outer levels will have energies close to that of the quantum. It will be realized that there is here an admirable method of in vestigating the inner structnre of the atom. By studying the energies of the different groups of photoelectrons and finding how much less these energies are than that of the original quantum, we can ascertain the orbit from which each electron comes, and the general character of its interaction with light under given con ditions.
The general type of apparatus which is used in these investiga tions is shown in fig. 2. The source of the photoelectrons may be either a thin foil on to which X-rays are fired from outside the box, or a wire coated with radioactive material, which, as will be described later, is a copious emitter of photoelectrons. Above the source is a slit which allows a beam of the electrons to pass. This apparatus is placed in an evacuated box between the poles of a large electromagnet, and on application of the magnetic field the beam of electrons is bent round into circular paths. By this
process it will be analysed into its different velocities, since the fastest electrons will describe the largest circles, and the slowest electrons the smallest circles. A record of the velocity spectrum of the photoelectrons is obtained by a strip of photographic plate. This is frequently referred to as the focussing method since, as can be seen from fig. 2, the rays leaving the source within quite a large solid angle are concentrated in one spot on the photo graphic plate. A reproduction of a photograph was taken recently in this way by Prof. H. H. Robinson.
This type of apparatus does not give lines sharp on both sides but only on the high velocity side, and a series of such lines can be seen on the photograph. These show the various groups of elec trons ejected from gold atoms by the characteristic K X-rays of copper (Cu On the original photograph it is possible to dis tinguish no less than 13 groups, some of which are partially super imposed. All of these are due to the absorption of the same fre quency radiation, and the difference in the energies of the groups is due to the multiplicity of the electron levels in gold.
This method was first tried by Robinson and Rawlinson in 1913, and owes its development largely to M. de Broglie. Since the frequency of the X-rays used is known from crystal measure ments, and the energies of the electron groups can easily be ascer tained from the geometry of the apparatus and the value of the magnetic field, it has been possible to verify Einstein's law with considerable accuracy. The accurate measurements carried out by Robinson have confirmed the truth of this equation to be at least one part in 300. It should, however, be emphasized that the importance of the method is that it provides an admirable means of investigating the electron levels in the atom, and from the in tensity of the photographi traces it is possible to estimate the probability of absorption of the radiation by each of the separate levels.