The usual source of positive rays, as has been stated, is the ordinary discharge in gases at low pressures. For examination by this process a substance must itself be volatile or capable of yielding a stable compound which can be introduced as a vapour into the discharge tube. The majority of elements are metals and do not behave in this manner. To obtain mass rays of these other methods must be used. As long ago as 1906 it was observed that halogen salts when heated and used as anode in a discharge tube behaved in a very remarkable manner giving off pencils of coloured rays. Thus the salts of lithium produced brilliant red streamers corresponding to the well-known red line of the lithium spectrum. These were called "Anode Rays" and proved to be positively charged atoms of the metals employed. These rays are usually too slow to be suitable for direct analysis by the mass-spectrograph but they can be made so by the device illustrated in fig. 6, called the method of "accelerated anode rays." The discharge providing the rays takes place between the anode, containing halogen compounds, and a subsidiary cathode as shown. The slow rays passing through the latter are accelerated by introducing between it and the cathode of the mass-spectro graph a kenotron which will only allow a current of a value desired to pass. Now if the discharge tube becomes too hard the cathode rays from the subsidiary cathode will increase in energy heating the anode and softening the tube. If, on the other hand, the dis charge becomes too soft the energy is absorbed in the kenotron, that in the cathode rays decreases and the anode cools down hardening the discharge. Also, whatever the state of the discharge tube, the energy of the rays reaching the mass-spectrograph is roughly constant, for this is governed by the watts in the high tension discharge and the current allowed to pass through the kenotron. By means of this device applied to the original mass
spectrograph the isotopic constitution of a large number of metallic elements was demonstrated. (See IsuroPEs.) Mass-spectrographs of High Precision.—The accuracy and resolving power of the original mass-spectrograph have since been greatly exceeded. Costa, in 1925, described an instrument capable of an accuracy of I in 3,000 by which he compared the masses of the atoms of lithium and other light elements by the method of bracketing. The latest model now in use by the writer in the Cavendish laboratory has a resolving power of i in 600, more than sufficient to resolve the mass lines of the isotopes of any known element, and, when suitable systems of measurement are used, an accuracy of i in eo,000. This instrument has been used to meas ure the very small divergencies from the whole number rule. (See ISOTOPES.) In conclusion, it may be stated that the discovery of positive rays gave a very valuable weapon into the hands of the physical investigator. The study of their analysis is less than 20 years old, yet already it has established facts of revolutionary importance and has provided a means of measuring the weights of atoms, free from ambiguity and of an accuracy equal to that of the finest chemical methods.
See J. J. Thomson, Rays of Positive Electricity (2nd ed.) ; F. W. Aston, Isotopes (2nd ed., 1924) and Bakerian Lecture, Roy. Soc. Proc. A. 115 (1927) ; W. Wien, Kanalstrahlen (5923). (F. W. A.)