Friday, May 7, 2010

Divisible Part 2


During the 19th century physicists developed a concept that electrical charge was an indivisible quantity. In 1874 this idea was championed by Irish physicist George Johnstone Stoney (1826-1911). However he incorrectly believed that these charges were always attached to the atom and could not be removed. Experimental work by William Crookes (1832-1919) in 1879 first demonstrated that these charge carriers were negatively charged. He believed that these charges were rays being emitted by the cathode of his discharge tube experiment. But in 1895, French physicist Jean Perrin (1870-1942) established that these 'rays' were in fact particles. He done this by adding a small wheel assembly to the Crookes' Experiment which moved upon bombardment of the cathode's stream.

Crookes Tube

The discovery of the electron as it became known was attributed to J. J. Thompson (1856-1940) but it was known for years that charge was carried in some form by a wave or particle. However Thompson with his colleagues did establish the charge to mass ratio for the electron.

Not only did Thompson establish the charge/mass ratio, he also demonstrated that electrons were emitted by radioactive, heated matter and illuminated matter. This was also independently verified through experimental work by Henri Becquerel (1852-1908), who showed that certain radiation emitted from substances could be deflected in the same manner as electrons and also have the same charge/mass ratio.

It was not until the early 20th century that the mass of the electron was found. In an experiment developed by American physicist Robert Millikan (1868-1953) in 1909, a charged droplet of oil was suspended in an electric field so that it remained stationary in the gravitational field. This was repeated for various charges and it was found that these were all integer multiplies of the same value. From this work the charge on the electron was established, which in turn gave rise to the value for the mass of the electron.

The mass of the electron was surprising, being only a small fraction of the mass of the lightest known element at the time, hydrogen. This was the first sign that the atom was not some solid piece of matter and gave rise to the view that electrons were some form of component of an atom.

Based on this discovery, Thompson put forward his model for the structure of the atom. In his model the atom was akin to a plum pudding. The electrons were scattered throughout the pudding whose charge was positive, therefore making it electrically neutral.


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