J. J. Thomson at the cavendish laboratory: The history of an electric charge measurement

Summary

J. J. Thomson's discovery of the negatively charged corpuscle in 1897 is customarily regarded as the discovery of the electron. Thomson, however, did not immediately equate the charge of his corpuscle with the unitary charge, that is the ‘electron’, first proposed by Stoney in 1874. The aim of this paper is to clarify the means by which this identification was eventually made. To do this the work carried out by Thomson and his students at the Cavendish Laboratory between 1897 and 1899 has been examined. From this reconstruction it emerges that, following his work on the mass-to-charge ratio of the corpuscle in 1897, Thomson and his school initiated and developed a series of techniques for measuring the charge of the ions. These techniques could not be used directly to measure the charge of the corpuscles because of the conditions required to produce them. Thomson therefore sought some other phenomenon that could be interpreted in terms of corpuscles and which allowed exploitation of the new charge-measuring techniques. He found such a phenomenon in the photoelectric effect, which allowed the measurement of both the charge and the mass-to-charge ratio of the corpuscle to be made. These measurements showed the charge of the corpuscle to be close to that assigned to the ‘electron’, and the two entities gradually became equated with each other.