Particle spin

Physicists realized that the fourth quantum number indicates particle spin, which can be +½ or -½. This concept was applied to particles like electrons and later found to apply to other particles as well.
 George Eugene Uhlenbeck, Samuel Abraham Goudsmit Netherlands 1925 Particle spin

Physicists realized that the fourth quantum number indicates particle spin, which can be +½ or -½. This concept was applied to particles like electrons and later found to apply to other particles as well.