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12v is supplied to the positive terminal of each coil. When the engine isn't running the negative side is not grounded, so the 12v appears there too. When the engine is running, the negative side is grounded for a length of time known as "dwell" to get current flowing in the coil primary. When it's time for a spark, the negative side of the coil is ungrounded to cause the voltage spike in the primary that's amplified by the turns ratio of the coil and sent the spark plug.
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