## § Einstein-de Haas effect

I learnt of this from hacker news. This is some crazy experiment that shows
that the 'quantum angular momentum' (spin) and the 'classical angular momentum'
need to be conserved *together* for physics to work out:
There's an experiment that transfers that angular momentum all the way up to macroscopic levels. By magnetizing a cylinder of iron, all the spins start pointing in the same direction. By conservation of angular momentum, the cylinder itself has to start spinning in the opposite direction. I'm very fond of this experiment, because it magnifies a strange quantum phenomenon to the classical level.

So, my understanding of the experiment is:
- classical angular momentum and quantum angular momentum are related.
- quantum angular momentum is decomposed into spin and orbital angular momentum.
- for something like iron, spin is 96% of magnetization
- angular momentum is proportional to magnetization
- So, the experiment measures the
*spin* (mostly) in terms of the classicalspinning of the cylinder.

#### § References