SSoon after Thomson's work had been published, the molecular model of cathode rays was universally accepted by scientists. However his model, according to which molecules of cathode rays are contained in all atoms and constitute one of their components, was not met with enthusiasm (many scientists rejected it).
In following years Thomson was trying to determine more precisely the q/m constant of electrons. Many discoveries made in that period evidenced universal existence of those molecules in nature.
In 1896 a Dutch scientist Pieter Zeeman placed a source of light between poles of an electromagnet and observed spectrum dispersing perpendicular to the direction of the magnetic field. In first experiments he failed to observe anything but when he increased the intensity of the magnetic field, Zeeman observed a slight widening of spectral lines. The scientist turned to Anton Lorentz for help in explaining the discovered phenomenon.
Assuming that within an atom there are some small, charged and moving on orbits molecules which emit light, one can expect that under the external magnetic field those orbits will be altered (lengthened or shortened). The change of orbits depends on the q/m ratio of the moving molecules. Thus, if the widening of spectral lines is known, it is possible to determine that ratio. Zeeman and Lorentz calculated that q/m of those molecules is approximately equal to q/m of Thomson's electrons.
Soon after that discovery, Zeeman used a stronger magnetic field and noticed that the widening of a line was actually its diffraction into three components (the same discovery was made parallel by Sir Oliver Lodge). The next discovery Zeeman made was when he observed the polarisation of light dispersing parallel to the field. When analysing results of the experiments, the scientist came to a conclusion that this light must have been emitted by negative molecules moving inside atoms. That was the only explanation of the observed experimental facts. Thus everything pointed to the fact that electrons moved on orbits inside atoms.
In 1887 Hertz discovered the phenomenon of emission of negatively charged molecules from a metal under ultraviolet light. The number of emitted molecules was very small and for a long time it was impossible to determine their charge to mass ratio. As late as in 1899 Thomson made such an experiment.
In order to calculate the q/m ratio of molecules extracted from metal under ultraviolet light, the scientist constructed a special device (fig. 1). Light hit a metal plate at a certain angle. There was another plate charged positively over that plate, placed at a certain distance. Light hitting the plate extracted molecules from it. The molecules were accelerated by the electric field towards the charged plate. A magnetic field parallel to the plates bent the route of the molecules. At some adequately high intensity of the field all negatively charged molecules moved in a circle and returned to the plate from which they had been extracted. Using a sensitive electrometer attached to the positively charged plate, Thomson managed to determine the intensity and the q/m ratio of those molecules. That value equalled 0.76*1011 coulombs per kilogram and hence it was very close to the q/m of molecules of cathode rays. That supported the view that those molecules are electrons.
| In 1883 Thomas Alva Edison (1847-1931 - a particularly gifted American scientist) discovered that fibre, incandescent due to the electric current flow, emits some negatively charged molecules. In 1899 Thomson carried out an experiment the course of which was similar to the experiment of molecules extracted by light. The scientist managed to figure out the ratio of the charge to the mass of molecules emitted by incandescent metal, which equalled 0.87*1011 coulombs per kilogram. Hence most probably also those molecules must have been electrons. |
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At the end of the 19th century the phenomenon of radioactivity was investigated. It was known that radioactive substances emitted certain molecules of negative charge called "beta rays". In 1900 Becquerel calculated their charge to mass ratio in a way similar to the one used by Thomson for cathode rays. It turned out that the ratio equals 1*1011 coulombs per kilogram.
During the last four years of the 19th century scientists discovered that the negative molecules which are emitted in various physical processes are electrons. As Zeeman claimed, electrons are contained within atoms and so they are more elementary components of the matter than atoms. The estimate q/m value for electrons was calculated to equal 1*1011 coulombs per kilogram. However, it was not known what the values of q and m of an electron were. Attempts at figuring those values out were the focal point of many researchers in the first decade of the 20th century.
