The coil operates according to Faraday's Law of electro-magnetic induction.
As the high tension voltage in the secondary winding builds up, a particular voltage level is reached when the spark plug gap suddenly becomes conductive, allowing the spark to occur. This is known as the “plug firing voltage”.
The level of plug firing voltage depends on many factors, such as:
When the spark reaches the point where a high intensity current is flowing in the secondary circuit, the voltage drops to a much lower spark voltage level, sufficient to sustain the spark. This gives the mixture the opportunity to ignite during the duration of the spark.
