This Day In History: April 30

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On April 30, 1897, British physicist J.J. Thomson announced his discovery that atoms were made up of smaller components. This finding revolutionized the way scientists thought about the atom and had major ramifications for the field of physics. Though Thomson referred to them as "corpuscles," what he found is more commonly known today as the electron.

Mankind had already discovered electric current and harnessed it to great effect, but scientists had not yet observed the makeup of atoms. Thomson, a highly-respected professor at Cambridge, determined the existence of electrons by studying cathode rays. He concluded that the particles making up the rays were 1,000 times lighter than the lightest atom, proving that something smaller than atoms existed. Thomson likened the composition of atoms to plum pudding, with negatively charged "corpuscles" dotted throughout a positively-charged field.

The plum pudding analogy was disproved by Ernest Rutherford, a student and collaborator of Thomson’s, in Thomson's lab at Cambridge in 1910. Rutherford's conclusion that the positive charge of an atom resides in its nucleus established the model of the atom as we know it today. In addition to winning his own Nobel Prize, Thomson employed six research assistants who went on to win Nobel Prizes in physics and two, including Rutherford, who won Nobel Prizes for chemistry. His son, George Paget Thomson, also won a Nobel Prize for his study of electrons. Combined with his own research, the network of atomic researchers Thomson cultivated gave humanity a new and detailed understanding of the smallest building blocks of the universe.