Bakerian Lecture.— A new mass-spectrograph and the whole number rule

Abstract The original mass-spectrograph was set up in the Cavendish Laboratory in 1919. Its resolving power was sufficient to separate mass lines differing by about 1 in 130 and its accuracy of measurement was about 1 in 1000. These capabilities sufficed to determine with fair certainty the isotopic constitution of over 50 elements, and to demonstrate that, with the exception of hydrogen, the masses of all atoms could be expressed as integers on the scale O = 16 to one or two parts in one thousand. An account of these researches has already been published. The instrument itself was not actually dismantled until March, 1925, but some years before then it had been realised that for advance in two directions of fundamental importance, namely, the resolution of the mass lines of the heavier elements and the measurement of the divergences from the whole number rule, a considerably more powerful instrument would be required. The Department of Scientific and Industrial Research, to whom I should like now to express my thanks, provided a liberal grant to defray the expenses of construction, and preliminary work on the instrument was commenced in 1921. The application of the method of accelerated anode rays led to an unexpected lengthening of the useful life of the original apparatus so that it was considered best to hold up construction of the new one in order that the final design might have the advantage of all accumulated experience. In the meanwhile one of its objects, the measurement of the divergences from the whole number rule, had been attacked by Costa in Paris, using a mass-spectrograph of his own design capable of an accuracy of 1 in 3000. This admirable piece of work will be referred to later. The accurate determination of these divergences is of fundamental importance since it is one of the few avenues by which the problem of the structure of the nuclei of atoms can be approached, and it was worth making every effort to push the accuracy of analysis to its extreme limit. It was finally decided that the increase of resolution could best be obtained by doubling the angles of electric and magnetic deflection, and sharpening the lines by the use of finer slits placed further apart, in addition special methods were considered for the necessary increase of accuracy in measurement. After numerous setbacks all these objects have been successfully carried out. The new instrument has five times the resolving power of the old one, far more than sufficient to separate the mass lines of the heaviest element known. Its accuracy is 1 in 10,000 which is just sufficient to give rough first order values of the divergences from whole numbers.