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American Institute of Physics: Discovery of the Electron
Use this resource to read about the "Experiments by J.J. Thomson in 1897 [which] led to the discovery of a fundamental building block of matter." This exhibit is organized into several sections, including: "Mysterious Rays" and "1897...
Nobel Media AB
The Nobel Prize: J. J. Thomson Nobel Lecture: Carriers of Negative Electricity
At this site from The Nobel Foundation you can download and read Joseph John Thomson's Nobel Lecture, "Carriers of Negative Electricity" (in pdf format).