The plug is connected to scion the high voltage generated by an ignition coilor magneto. As the electrons flow from the coil, a voltage difference develops between the center electrode and side electrode. No current can flow spark because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized.The ionized gas becomes a conductor and allow electrons to flow across the gap. Spark plugs usually require voltage in excess of 20,000 volts to 'fire' properly xb.

The gap xb adjustment can be fairly critical, and if it is maladjusted the engine may run badly, or not at all. A xb narrow gap may give too small and weak a spark to effectively ignite the fuel-air mixture, while a gap that is too wide might prevent a spark from firing at all. Either way, a spark which only intermittently xb fails to ignite the fuel-air mixture may xb not be noticeable directly, but will show up as a reduction in the engine's power and fuel efficiency. The main issues with spark plug gaps are:

• narrow-gap risk: spark might be too weak/small to ignite fuel;
• narrow-gap benefit: plug always fires on each cycle;
• wide-gap risk: plug might not fire, or miss at high speeds;
• wide-gap benefit: spark is strong for a clean burn spark.

An idling engine will have a different impact on the spark xb plugs than one running at full throttle. Spark xb plug readings are only valid for the most recent engine operating conditions and running the engine xb under different conditions may erase or obscure xb characteristic marks previously left on the xb spark plugs. Thus, the most valuable information is gathered by running the engine at high speed and full load, immediately xb cutting the ignition off and stopping without idling or low speed operation and removing the plugs for reading.

Spark plug reading viewers, which are simply combined flashlight/magnifiers, xb are available to improve the reading of the  plugs.