This book tells the curious story of an unexpected finding that sheds light on a crucial moment in the development of physics: the discovery of artificial radioactivity induced by neutrons. The finding in question is a notebook, clearly written in Fermi's handwriting, which records the frenzied days and nights that Fermi spent experimenting alone, driven by his theoretical ideas on beta decay. The notebook was found by the authors while browsing through documents left by Oscar D'Agostino, the chemist among Fermi's (...) group. From Fermi's notes, they reconstruct with skill and expertise the detailed timeline of the critical days leading up to his vital discovery. While much is already known about the road that led Fermi to his important result, this is the first time that it has been possible to reconstruct precisely when and how the initial evidence of neutron-induced decay was obtained. In relating this fascinating story, the book will be of great interest not only to those with a passion for the history of science but also to a wider audience. (shrink)

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. (shrink)

In this paper we analyse the approach to interpreting atomic spectra in the framework of classical physics from the discovery of the electron in 1897 to Bohr's atomic model of 1913. Taken as a whole, efforts in this direction are part of a remarkable intellectual endeavour in which the classical theoretical framework seems to have been exploited to its full potential. By demonstrating the limits and weaknesses of classical physics in solving the problem of spectral emissions, these attempts opened the (...) way to a complete break from traditional thought and the introduction of the new quantum ideas. (shrink)

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 , 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. (shrink)

Until now studies on the historical development of atomic spectroscopy have focused on three main aspects-its first applications as a method of chemical analysis, the formulation of spectral laws , and the rise of the old quantum theory. These developments of spectroscopy were based on the same assumption: the invariance of the atomic spectrum after fixing the chemical element . This paper shows that running alongside these lines of research there was another, no less important area of study based on (...) the negation of this assumption. The focus of these latter studies was the behaviour of atomic spectra as a function of different working conditions. English astrophysicist Sir Norman Lockyer played a central role in this research. While studying the effect of temperature on spectral lines and comparing stellar spectra with laboratory spectra, he discovered a new class of spectral lines . In an attempt to explain the origin of these lines, Lockyer, at the beginning of 1897 , hypothesized the 'dissociation' of the chemical elements into simplified forms of matter, named proto-elements. Lockyer's studies are mentioned by Thomson in the paper which introduces the corpuscle for the first time. In this paper we have tried to reconstruct the history of Lockyer's proto-elements and highlight the links between them and Thomson's corpuscle. Our analysis has shown that not only the famous studies on discharges in rarefied gases but also Lockyer's stellar spectroscopy played an important role in the work of J. J. Thomson, convincing him that the atom was complex and divisible. (shrink)

When at the end of the 1900s Planck introduced the constant h into the black-body radiation law together with constant k, he provided no explanation of either its meaning or why it had that particular value. He simply introduced it. In reality the history of the constant was far from straightforward. Planck was confident enough to introduce it like this because he had been working on the question for over a year. In this paper we reconstruct the process that began (...) with the first two constants (c' and C. (shrink)

The first models for an electromagnetic theory of dispersion are presented and an attempt is made to demonstrate the important role played by study of this phenomenon at the end of the nineteenth century. As well as indicating the need to have a better understanding of the microscopic properties of matter, dispersion also contributed to the discussion over the nature of X-rays and was fundamental for introduction of Lorentz's electron theory.