How Electron Spin Was Discovered

In 1887, physicists Albert Michelson and Edward Morley discovered a new phenomenon that would come to be known as the fine structure of spectral lines. These spectral lines split into two or more components, and other effects like the Zeeman and Stark effects also showed cases where spectral lines split. The quest to answer why this happened took almost 40 years and would end with the discovery of a new characteristic of an electron known as "spin".