Between 1855 and 1860 Heinrich Geissler (1814-1859) developed a mercury diffusion pump producing high vacuums. He produced electric discharge between electrodes sealed in evacuated glass tubes. Julius Plücker (1801-1868) noted fluorescence on the glass when an electric spark passed through Geissler tubes. The fluorescence could be moved with a magnet. This was consistent with the idea that moving bodies carrying electric charge should be deflected in a magnetic field. But such charged bodies should also be deflected by metal plates charged with electricity. But no such deflection could be demonstrated.
J.W. Hittorf noted that the electric discharge in Geissler tubes produces shadows behind obstacles suggesting that the sparks came from the negative electrode.
In 1874 William Crookes turned a small paddle wheel with the discharge. He believed the rays were negatively charged material, perhaps a fourth state of matter, called Crookes rays by many.
In 1883 Svante Arrhenius (1859-1927) presented to the Swedish Academy of Sciences his theory that neutrally charged salts dissociate when dissolving to form electrically charged atoms or groups of atoms called ions. The ions explained electrical conduction of solutions of salts, anomalies in osmosis, and similarities in the heats of neutralization. This explanation eliminated much of the confusion about the properties of solutions and indefinite compounds that had seemed to Mendeléeff and others to refute the atomic theory. The atomic theory continued to remain controversial until after 1900.
After Arrhenius introduced his theory about charged ions, many suspected that the current in Geissler's tubes is carried by charged atoms similar to the current carried in solutions.
Eugen Goldstein (1850-1930) proposed the negative (-) radiation be called cathode rays. Behind holes bored in the cathode he found opposite charged (+) radiation in 1886 for which he proposed the name canal rays. In 1891 George Johnstone Stoney (1826-1911) proposed the unit of charge should be called an electron.
In 1897 Wilhelm Wien (1864-1928) measured the mass of the positively charged canal rays, finding they have the mass of ions. Thus gases, like solutions, do use ions to conduct electricity.
In 1897 Joseph John Thomson (1856-1940) at Cambridge, suspected that residual air stuck to the internal tube parts might shield the rays from electric fields thus blocking the expected attraction or repulsion by charged plates. Thomson baked the tubes to release residual gases prior to final evacuation. He was then able to deflect the cathode rays by an electric field. Using both an electric field and a countering magnetic field, Thomson was able to just cancel the two forces, then using the equations for both electric and magnetic force, he calculated the speed of the rays, and the ratio of their electric charge to mass (q/m). The speed was near that of light. The q/m ratio was consistent with a mass less than 1/1000 that of the lightest atom, Hydrogen. Thomson proposed to name the carrier of cathode rays the electric corpuscle, but Goldstein's name electron became more popular.
J.J. Thomson showed that identical electrons were obtained from every kind of metal electrode he tried. Thomson concluded the electron is probably a fundamental part of all atoms.
J.J. Thomson and William Thomson (Lord Kelvin) proposed a model for atoms suggesting that atoms are built like plum pudding. Negative electrons are like raisins evenly distributed through positively charged pudding. (Plum pudding is a common English steamed bread pudding well mixed with various nuts and fruits)
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| created 23 March 2002 latest revision 4 November 2006 |
by D Trapp |  |
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