Came across this fascinating post about Magnetic Amplifiers and thought CENTOR might find it as interesting as I did.
The basic premis is that varying the core in an inductor can control the flow of alternating current.
Here is a link to the article: https://www.nutsvolts.com/magazine/article/the_magnetic_amplifier
While diving down the magnetic rabbit hole I also came across the Marconi Magnetic Detector.
Link to youtube video: youtu.be/4CAxWoKD-fw which is unfortunately not very high quality.