I've written before about the Einstein-deHaas effect - supposedly Einstein's only experimental result (see here, too) - a fantastic proof that spin really is angular momentum. In that experiment, a magnetic field is flipped, causing the magnetization of a ferromagnet to reorient itself to align with the new field direction. While Einstein and deHaas thought about amperean current loops (the idea that magnetization came from microscopic circulating currents that we would now call orbital magnetism), we now know that magnetization in many materials comes from the spin of the electrons. When those spins reorient, angular momentum has to be conserved somehow, so it is transferred to/from the lattice, resulting in a mechanical torque that can be measured.
Less well-known is the complement, the Barnett effect. Take a ferromagnetic material and rotate it. The mechanical rotational angular momentum gets transferred (via rather complicated physics, it turns out) at some rate to the spins of the electrons, causing the material to develop a magnetization along the rotational axis. This seems amazing to me now, knowing about spin. It must've really seemed nearly miraculous back in 1915 when it was measured by Barnett.
So, how did Barnett actually measure this, with the technology available in 1915? Here's the basic diagram of the scheme from the original paper:
There are two rods that can each be rotated about its long axis. The rods pass through counterwound coils, so that if there is a differential change in magnetic flux through the two coils, that generates a current that flows through the fluxmeter. The Grassot fluxmeter is a fancy galvanometer - basically a coil suspended on a torsion fiber between poles of a magnet. Current through that coil leads to a torque on the fiber, which is detected in this case by deflection of a beam of light bounced off a mirror mounted on the fiber. The paper describes the setup in great detail, and getting this to work clearly involved meticulous experimental technique and care. It's impressive how people were able to do this kind of work without all the modern electronics that we take for granted. Respect.
4 comments:
Very cool! I really wish we could do more to bring back this style of science, low-budget but high in cleverness. I see this style of science with Youtube amateur scientists (Ben Krasnow - homemade SEM, Sam Zeloof - homemade IC chips), but Doug what can we do to promote this in research labs and universities? When I was an undergrad at Princeton our labs weren't much more than regurgitation of the lab manuals. And during grad school the pressure to get results meant that spending money was better than spending time.
Anon, good question. The various maker-space/exploratory learning approach to lab instruction are a way to try this, but for a lot of students they're just a source of painful frustration. (And I've had colleagues at various times say that MacGyver-esque labs don't necessarily teach much physics, but it's not clear that's really the point of undergrad labs. Instructionally I think we're trying to teach techniques, care/logic, and resourcefulness more than confirming theoretical models.) Regarding grad school, there is always room for innovation in technique, but I get your point. My thesis adviser built his own dilution refrigerator while a grad student at Cornell. These days b.c of the pressure for quick results it would be absolutely self-defeating for a new PI to have students spend time building complex apparatus if it was possible instead to get going doing science with commercial equipment. Many students don't have the same gear-head building skills that used to be cultivated, but on the other hand probably know enormously more about computer control of equipment and analysis of large data sets.
This isn't really related, but since Doug you are a true to heart nano-guy, I wonder if you could do a blog post about the "Graphene Nano Spray" for cars that is getting popular on social media?
https://www.goldeagle.com/product/303-graphene-nano-spray-coating/amp/
Anon, I don’t think it’s a great idea to spray bullshit onto a car....
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