Atoms and their Packing Fractions¹

Abstract

THE original mass-spectrograph was set up in the Cavendish Laboratory in 1919. Its resolving power was suffint 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 more than fifty elements, and to demonstrate that, with the exception of hydrogen, the masses of all atoms could be expressed as integers on the scale 0=16 to one or two parts in one thousand. 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 was required and has now been constructed. The increase in resolution is obtained by doubling the angles of electric and magnetic deflexion, and sharpening the lines by the use of finer slits placed farther apart. 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 enough to give rough first order values of the divergences from whole numbers.