The initial voltage ot the spark plug causes breakdown voltage to ionize the air-fuel charge in the gap to become conductive. There are various designs of the Ground Electrode configurations that are not standard. These include a ground strap over the center electrode, multiple electrode and various electrode shapes. For the purpose of discussion, refer to this diagram;
The spark strike is always a series of quick single burts which generally strikes at an edge of the ground. The strike always develops a negative space charge which prevents the following strike to hit at that same point. In the case of the side electrodes, the following strike will be at another ground tip. In the case of the ring electrode, the strikes will rotate. For the case of the single eletrode strap, the successive strikes will be random as a result of the charge on the ground electrode strap. In the case of the short ground over the center electrode, the strikes will follow along the edge. The concept is to produce forward flame.
This is an important concept to understand. There can only be one spark at a time, and the resulting potential increase at strike point requires the spark to move to another location … lighting really doesn’t strike in the same place twice, at least not for a little while. So multiple ground configurations do not get two sparks, they simply get a spark that vibrates between forks or between electrodes. Is this an improvement or a gimmick?
The Sonic Spark Plug is unique in its strike patern. The strikes target to the center hole in the ground, and rotates along the inner edge of the hole. The ground hole is an ultra high frequency whistle which puts a spin on the flame which is projected frontally in vibration.
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