# Quantum Spin intrinsic once inside a magnetic field

In a few textbooks I've read that Quantum Spin (which is indeed intrinsic) only 'reveals' itself in the presence of a magnetic field. This seems very odd (but let's be honest, it's quantum mechanics) and I was wondering, is there an experiment that validates this? I've looked and haven't been able to find anything. The only experiment that I could think of is a Stern-Gerlach-like device where $B=0$, but $d B/d z \neq 0$.

• If $d B/d z \neq 0$ doesn't that mean $B \neq 0$? – jim Feb 13 '18 at 21:11
• @jim No. A Quadrupole magnet set-up has $B=0$ in the middle, but is definitely not $dB/dz=0$. I'm sure that there are other setups as well. – Josh Feb 13 '18 at 21:45
• In a basic and naive Stern-Gerlach device (one narrow pole and one wide pole) the magnetic field isn't zero. In fact it is usually pretty strong, it's just that the particles are neutral and therefore don't experience a force do to the field, but their dipole-moment does experience a force due to the spatial variation of the field. – dmckee --- ex-moderator kitten Feb 13 '18 at 23:13