THOMSON,J.J. AT THE CAVENDISH-LABORATORY - THE HISTORY OF AN ELECTRICCHARGE MEASUREMENT

J. J. Thomson's discovery of the negatively charged corpuscle in 1897 is customarily regarded as the discovery of the electron. Thomson, how ever, did not immediately equate the charge of his corpuscle with the unitary charge, that is the 'electron', first proposed by Stoney in 18 74. The aim of this paper is to clarify the means by which this identi fication was eventually made. To do this the work carried out by Thoms on 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, Thomso n and his school initiated and developed a series of techniques for me asuring the charge of the ions. These techniques could not be used dir ectly to measure the charge of the corpuscles because of the condition s required to produce them. Thomson therefore sought some other phenom enon that could be interpreted in terms of corpuscles and which allowe d 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 b e made. These measurements showed the charge of the corpuscle to be cl ose to that assigned to the 'electron', and the two entities gradually became equated with each other.