A pinball coil is just a "coated" or "painted" piece of wire. The type of wire is commonly referred to as "magnet wire" outside of the pinball world. This is the type of wire used for 99% of all coils in everything in our world.
As many others have suggested, the pinball coil (made of a coated piece of wire) does not care how the electricity flows. Only the implementation of a diode into the circuit matters. This is one major reason why some manufacturers removed the diodes from the playfield/coils, and moved them to the head, or on an additional daughter board with plugs.
This way, it does not matter which wire is connected to which side of the coil, since when using that type of design, all the coils would be without a diode.
THIS IS ALSO WHY THERE IS ALWAYS VOLTAGE ON ALL LUGS OF A COIL. The most common fallacy pinball people don't realize is that when a game is in play mode, all lugs of all coils ALWAYS have voltage. The GROUNDING of a coil is what determines a coil's actuation. Furthermore, the diode is what blocks that grounding action from affecting other components in that same circuit.
Now, most flipper coils have two unique coil windings with two diodes, bot those coils use all these same principals to work.
If all pinball owners trying to fix a coil problem knew these two simple facts and how to use a voltmeter, they would be able to fix a large majority of coil related pinball problems.
Now, more modern pinball designers like 90's B/W, Stern and JerJack all use slightly modified designs to "cycle" how long coils ground for... This happens in hundredths or thousands of a second and is controlled by the MPU in a game... Think of that as more of an electric determination for the time that a coil is ON, rather than a physical. It is VERY similar to the difference between an analog power supply and a switching power supply.
