Hi, I'm an hobbyist right now I'm into electric engines, simple engines for low voltage and low current applications; I have noticed that there are many variations about how and what an electric engine can do even considering only DC motors, but basically all the engines that I have examined up until now have something in common, and that is the presence of a coil, a copper coil, used as a conductor in the internals of the engine itself . I wonder if such twisted copper cables are the only way to induce current in a DC / AC engine, for example can you replace a coil with a ring of copper ? A cylinder ? What is the physical explanation that motivates the use of a coil and what makes it efficient compared to other shapes / solutions ? After all the pulse of current is given by the change in the magnetic flux, which basically refers to the rotating parts of your engine, up until now I haven't found any explanation as to why it's an optimal thing to use a coil rather than a cylinder, or a sphere, or any other piece of geometry in metal form . There are formulas that dictate the ins and outs of an optimal coil given the specs of an electrical engine ?