Other end is the constant +12V (black wire) and the other is the ignition live (green wire). At crank speed (~200rpm) a spark is given each 300ms. When you compare this with a common coil dwell time of ~5ms, you would see that at average the ignition live is only a few tenths of a volt short of the battery voltage.

tabetha, the coil ratio is not that important as it first seems. Remember that several tens of kilovolts are easily acchieved even with common coil ratios of less than 100.