Zeeman's discovery and the mass of the electron

Summary

In an article published recently in this journal, one of us reconstructed how in 1899 J. J. Thomson, after having measured the mass-to-charge ratio of the corpuscle (which became our electron), achieved a measurement of its charge and consequently an estimate of its mass, obtaining in this manner ‘direct proof of the existence of particles smaller than the hydrogen atom’. In this paper, starting with an analysis of Zeeman's first measurements on the widening of spectral lines in a magnetic field, we show that the existence of masses smaller than the atom was already a matter of discussion in early 1897, and that a qualitative estimate of the mass of the electron was made even before Thomson's measurements in 1899. In this process an important role was played by a research programme on the structure of matter carried out by Stoney in the latter part of the nineteenth century. This had in many aspects anticipated Zeeman's experimental observations, and its conclusions were in part formalized in the so-called Larmor theorem.