I don't know how this motor works, but it instantly reminded me of my mentor's vintage guitar amp with a field coil speaker[1]. The key idea was that copper was

I don't know how this motor works, but it instantly reminded me of my mentor's vintage guitar amp with a field coil speaker[1]. The key idea was that copper was cheaper than magnets, so they make an extra coil of wire on the speaker which generates a magnetic field. Then the voice coil drives the speaker by modulating that field. This article[2] seems like a decent introduction to the topic. The thing that struck me about the amp was how lightweight it was. While newer amps may use rare earth magnets (which pack more magnetic field per gram) vintage amps usually had to use heavy inefficient magnets. [1]: http://www.preservationsound.com/2010/10/the-field-coil-guitar-amplifier/ http://www.preservationsound.com/2010/10/the-field-coil-guit... [2]: https://www.edn.com/field-coil-speakers-obsolete-or-the-future/ https://www.edn.com/field-coil-speakers-obsolete-or-the-futu...