Prior to Thomson’s work it was thought that the atoms of each element were unique, invisible particles, or that atoms were made of subatomic particles that had significant mass compared to the atom as a whole. Thomson used Crookes tubes to disprove both of these ideas with his discovery of the electron, a negatively charged entity with a minuscule mass compared to that of the atom. Thomson applied a magnetic field to a discharge passing through a Crookes tube. He then used the data from this experiment to determine the mass-to-charge ratio of the particles emitted at the cathode. To his surprise, Thomson found that this ratio was constant regardless of the applied voltage. This implied that the particles emitted at the cathode were identical. Thomson predicted that these particles, which he called electrons, were a fundamental part of all elements of matter.

This demonstration allows students to simulate the results that Thomson would have observed for a variety of different atomic models.

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