J. J. Thomson

Sir Joseph John "J.J." Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British physicist and Nobel laureate. He discovered the electron and isotopes, and invented the mass spectrometer. He was awarded the Nobel Prize in Physics in 1906 for his discovery of the electron and his work on the conduction of electricity in gases. John Joseph Thomson in 1893 said: "There is no other branch of physics which affords us so promising an opportunity of penetrating the secret of electricity." 
In 1897, Thomson showed that cathode rays were composed of a previously unknown negatively charged particle of very small mass compared to its electric charge. It was later called the electron and was the first subatomic particle to be found. Thomson also found the first evidence for isotopes of a stable (non-radioactive) element in 1913, while studying anode rays (positive cations). 
In atomic theory he hypothesized that atoms were spheres of evenly spread positive charge, where an individual negatively charged electron resided. He later concluded there was more than one negatively charged particle in an atom.
Many of the young men who studied and worked with him at University of Cambridge also became famous physicists.