As daz c says.
The coil is just that a coil of wrapped around another coil of wire wire.
It will of course be positve until it receives the opposing current on other end negative, this then generates a strong magnetically induced current in seconday(inner windings HT), when the negative is taken away by ignition circuit the filed induced voltage seeks a path to earth through the leads/plugs, all happens very quickly.
The ration of turns of primary and secondary coils dictates the voltage output as input voltage fixed 12v.
tabetha