Thomson Experiment

From ArticleWorld

Before the discovery of electrons, it was believed that atoms were the fundamental building blocks of matter. When scientists began studying properties of atoms in large electric fields, they began to discover sub-atomic particles. In the 19th century scientists were fascinated by the behaviour of cathode ray tubes. It was observed that when an electric current was passed through a vacuum tube, a beam of glowing material could be seen. J. J. Thomson marked that the stream bent towards the positively charged plate which suggested that the atoms have been broken into charged constituents. In 1894, Thomson carried out a set of experiments to prove that the cathode produced a stream of sub-atomic particles carrying a negative charge.

The experiment

J. J. Thomson developed an experiment to investigate the properties of these negatively charged particles. He designed tubes containing electrodes with air evacuated from the tubes. These tubes were originally known as "Crookes tubes" named after the designer William Crookes. They later came to be known as "Cathode Ray Tubes".

High voltage electric current was passed between these electrodes. Ray-like emissions proceeded from the cathode electrode to the anode electrode. These emissions were named as Cathode Rays as they were emitted by the cathode electrode. J. J. Thomson devised special tubes to study the properties of these rays. A beam of Cathode rays was allowed to strike against the surface of a zinc sulfide coated screen. A spark of light was seen in place of the invisible rays. He then brought an electrical field comprising of a positive and a negative plate near the vicinity of the rays. Under influence of electric current, the path of rays deflected from the positive plate to the negative plate which showed that these rays possessed a negative charge.

Thomson also measured the charge-to-mass ratio of the cathode rays by measuring the deflection of the rays by a magnetic field. The charge-to-mass ratio was over a thousand times higher than that of a proton suggesting that the particles were either very light or very highly charged. The proportion of the charge and mass he calculated approximately was 1×1011 coulombs per kilogram.

Publication of the results

He published the result of his experiment in 1897. Thomson's experiments proved that the atom was divisible and consisted of sub-atomic particles. He concluded that the atom is a charged ball with negative charges inside. He named these particles corpuscles, which later were named as electrons. This idea existed until Earnest Rutherford built the planetary model of atom in 1911.