As seen in the course reader page 62, the set up was to have an atom beam of silver atoms going through a magnetic field, which would hit a collection plate on the other side.
What happened was that the atoms (which were in their ground state with their 1 unpaired electron) were accumulating on two different points, one up and one down, of the collection plate, when they were expecting the atoms to be evenly distributed between the two points. There's a 50/50 chance of the electron being in an "up" or "down" spin (+1/2 or -1/2 ms
), so the "up" and "down" spins of the electrons was what made the atoms accumulate in those two distinct points.
Ok so I found this video on Wikipedia so it may not be 100% reliable, but I think this demonstrates what they expected to happen vs what actually happened and why pretty well: https://en.wikipedia.org/wiki/File:Quan ... riment.ogv