This electronics video tutorial provides a basic introduction into mechanical relays. It discusses the working principle of these devices using physics.
Crystals and Electricity: https://www.youtube.com/watch?v=dO8zsgbunvY
The Gallon Crusher: https://www.youtube.com/watch?v=abXVQZlFPJg&t=1s
Homopolar Motor: https://www.youtube.com/watch?v=MEdAvKiFe9w&list=PL0o_zxa4K1BVZbm2aWCslec4SCeCduiqv&index=3
