1922 – England
‘At the end of the First War, the assistant of JJ THOMSON developed the mass spectrograph for measuring the comparative weights of atoms’
Whereas Thomson had used a discharge tube to measure the deflection of atomic particles passing through a hole in the anode, Aston refined the instrument by placing photographic plates in the path of the beams emerging through a hole in the cathode. These rays proved to be much harder to deflect from their course, implying that they were made of particles thousands of times heavier than electrons, with masses close to those of atoms. These particles were deflected in opposite directions to negative cathode rays, indicating that they carried a positive charge.
Hence hurtling in one direction down a discharge tube were cathode ray electrons occasionally colliding with the atoms of the rarefied gas filling the tube. Drifting in the other direction – much more sluggishly because of their larger mass – were positive gas atoms, or ‘ions’, stripped of an electron or two in the collisions.
Once perfected, this mass spectrograph offered a means of deciding the mass of these atoms to an accuracy of 1 part in 100,000. This was enough to distinguish the existence of different isotopes and to confirm that the ‘rule of thumb’ – that masses of atoms were roughly whole number multiples of the mass of hydrogen – was in actuality accurate.
What it had confirmed was that the fundamental building block had the same mass as the proton, or hydrogen nucleus. When the mass spectrograph was first devised, the proton was the only particle with the mass of a proton, as the neutron was yet to be described by JAMES CHADWICK.
When comparisons of atomic mass were made, the oxygen atom was chosen as the standard with a mass of 16.
Today carbon is used as the atomic mass standard with a weight of 12.
Using this standard it was discovered that although the ratios of atomic masses were indistinguishable from whole numbers, helium being 4, oxygen 16, the atomic mass of hydrogen was anomalously high, being 1.008. The conclusion as to why this should be so had been suggested in the nineteenth century, but before Einstein had found little support. After the famous paper of 1905, however, it was not unreasonable to suggest that when hydrogen atoms came together or coalesced to form other elements, mass was lost as energy.