I’m prepping a new course to teach this semester: undergraduate Electromagnetism II! I’m trying to put together some nice simple demos to illustrate principles in the class, and I’ll blog some of those that work and are interesting.
When Michael Faraday discovered the phenomenon of electromagnetic induction in 1831, paving the way for the complete unification of electricity and magnetism, he came up with a variety of experiments to demonstrate the effect.
One of them is now known as the Faraday disk, and it is very easy to construct — though I ended up buying one. My version is shown below.
All of the parts are visible in this photo. By turning a hand crank, one rotates a copper disk between a pair of magnets (the black disks), one generates an electrical current that runs from the outer edge of the disk to the central axis. Wires connected to these two points runs to the red and black plugs, through which one can measure the voltage difference generated. It isn’t a spectacular amount — I measured about 5 millivolts, max — but it demonstrates the phenomenon known as rotational electromotive force.
As we have noted, however, Faraday interpreted his disk experiment as electromagnetic induction, not rotational emf. It turns out that he was kinda wrong — but he was also kinda right! The explanation of this simple experiment involves some rather deep concepts in physics, and inevitably leads us to Einstein’s theory of special relativity.