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Joseph John Thomson

In 1884, at age 28, J.J. Thomson became Director of the Cavendish Laboratory at Cambridge University. No one was more surprised than Thomson who had been decried as a "mere boy." Nevertheless, this mere boy turned what he described as a "string and sealing wax laboratory" into the world's preeminent center for experimental nuclear physics. It has been said that Thomson, like Michael Faraday, was greater than his discoveries. However, those discoveries were far from insignificant. Thomson and his student Ernest Rutherford were the first to demonstrate the ionization of air by X rays. So fundamental is this phenomenon that the phrase "ionizing radiation" remains the most concise way to characterize the wide range of electromagnetic and particulate radiation emitted by atoms. Nevertheless, Thomson is best known for his investigations into the nature of "cathode rays," (i.e., electrons). By deflecting these "rays" with an electric field, something that had been done previously with a magnetic field, Thomson provided conclusive proof that they were negatively charged particles. He determined their mass to be one two-thousandth that of the hydrogen atom, the smallest object known at that time. Thomson was thus the first to identify the existence of subatomic particles. This opened the door to a new world of which his student, Ernest Rutherford, would later master, as well as provide his own significant contributions to nuclear physics. Later, Thomson demonstrated that the interaction between electrons and matter produced X rays and that X rays interacting with matter produced electrons. Although it would fail the test of time, Thomson is usually credited with the first "modern" model of the atom, the so-called "plum pudding" model. In it, he pictured a sphere of positive charges mixed together with an equal number of electrons (i.e., negative charges). For his theoretical and experimental investigations into the electron and the conduction of electricity by gases, Thomson was awarded the 1906 Nobel Prize in physics. Ironically, Thomson, who had characterized the material properties of electrons, would live to see his son George P. Thomson receive the Nobel Prize for experimentally confirming the wavelike properties of electrons.

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