You are going to do plugs too aren’t you?
A little coil theory here if you don’t mind.
A connection is made to apply 12vdc to a coil. Current starts to flow but it is opposed by the induction of the coil. The voltage slowly overcomes the induction and more current flows. Current flow increases with time. At higher RPMs, it may only have 1/5000th of a second (200 microseconds) to build up, but in electrical time, thats a lot.
The current only flows through the primary of the coil which has fewer windings so it has less inductance so current flow builds quicker. As current builds up in the coil, magnetic lines of force build up around the coil. It is the build up of these that causes the inductance.
Now when the engine needs a spark, the connection that started the current flow is opened. When this happens, the magnetic field begins to collapse. The collapsing field cuts across the windings of the coil and when a magnetic field moves around a conductor, current is induced to flow. When the field collapses, it not only cuts across the primary windings, it also cuts across the secondary windings inducing current in those as well.
There are about 100 times as many secondary windings as there are primary windings. That alone would cause the voltage induced to be 100 times greater than the applied voltage, but there is more. There is also a time factor involved in that the field collapses many times faster (1.2 - 4 microseconds) than it builds up because of the extra resistance caused by the spark plug. So not only does the voltage go from 12vdc to 1200 v pulse, it an rise as high as 40,000 volts.
The voltage is determined by Ohms Law. I wont go into that but basically, the higher the resistance in the secondary circuit (coil + wire + plug), the higher the voltage. If the plug doesn’t fire, usually around 26,000 volts, then the voltage will go higher until it can find a path. It can jump from a weak part in the insulation of the wires or boots or it can arc internally in the coil, damaging it.
This is why it is important to change the plugs as well, and to always change them according to the maintenance schedule.