Final answer:
The electrical circuit from the spark plug back to the magneto is completed by grounding through the case of the appliance. Grounding is important for safety reasons and to provide a path for excess charge to flow.
Step-by-step explanation:
The electrical circuit from the spark plug back to the magneto is completed by grounding through the case of the appliance.
In an electrical circuit, grounding is important for safety reasons and to provide a path for excess charge to flow.
The case of the appliance is connected to the earth/ground through a green earth/ground wire, forcing the case to be at zero volts.