![]() For this month’s column, I knew that I wanted to write about the 125th anniversary of the electron’s discovery, which for simplicity’s sake I pegged to Thomson’s lecture. Of course, history is always more muddled than that. Thomson is often hailed as the discoverer of the electron based on that lecture 125 years ago. Corpuscles are electrons, and the plum pudding model gave way to Ernest Rutherford’s nuclear model in 1911. Thomson, who merely endorsed the idea.Ĭorpuscles and pudding are not how we think about the structure of an atom today. The model also became known as the Thomson model, although its chief proponent was William Thomson (Lord Kelvin), not J.J. In Thomson’s analogy, negatively charged corpuscles were like raisins suspended in a positively charged cake, resulting in a neutral atom. This model of the atom became known as the “plum pudding” model, so named for the popular English dessert. Thomson described his experiments with cathode rays to verify the existence of these subatomic corpuscles. ![]() “The atoms of the ordinary elements are made up of corpuscles and holes, the holes being predominant,” he continued. Thomson, during a lecture at the Royal Institution in London, on 30 April 1897. Many thanks to MagLab physicist William Coniglio, the science advisor for this page, for his time and expertise.“We shall call such particles corpuscles,” announced the physicist J.J. The electrons in the cathode rays would deflect toward the positively charged plates, and away from the negatively charged plates. The results showed electrostatic deflection (as opposed to the electromagnetic deflection described above). ![]() In addition to experimenting with magnets, scientists also experimented to see what would happen if charged plates were positioned near the tube. (As you apply that rule, remember that the electrons in the cathode ray are travelling opposite the flow of conventional current.) Try flipping the magnet by checking the Flip Magnet box, and observe how the beam then deflects in the opposite direction. That rule describes how a charged particle (our electron) moving in a magnetic field will be deflected by that field at a right angle to both the field and to the direction of the particle. What happens in the tube is a consequence of the Lorentz Force, which is explained by the left hand rule. ![]() Thomson would determine that the molecules hypothesized by Crookes were actually negatively charged subatomic particles that he called corpuscles, but which were eventually named electrons. William Crookes experimented with cathode rays and magnets in a similar manner, and his observations on the deflection of the rays by magnetic fields led him to conclude that they were composed of negatively charged molecules. Observe the effect of a magnetic field on cathode rays by using the Magnet Position slider to move a horseshoe magnet (its north pole facing you) so that its poles straddle the cathode ray tube. These electrons, or cathode rays, are passed through a small opening near the cathode and then travel in a straight line toward the anode, passing through a fluorescent screen positioned between the cathodes that allows you to see the path of the electrons. A high voltage is transmitted to the cathode ray tube, inducing the cathode to emit electrons – essentially an electrical current. The tube illustrated in the tutorial contains a negative electrode ( Cathode) at one end and a positive electrode ( Anode) at the other. Scientists used special vacuum tubes, such as the Crookes tube and the cathode ray tube, to study this phenomenon.
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