Spark Plug: Description and Operation

Spark Plug Cutaway View:

Spark plugs allow the high-Voltage secondary current to arc across a small air gap, for the purpose of igniting the air-fuel charge in the combustion chamber.
The spark plug provides a path for the high current of the secondary circuit to flow to ground. The only paths for this current are through the ground electrode and center electrode across the spark gap.

Spark Plug Temperature:

The temperature of the spark plug is determined by the length of the insulator and the size of the heatsink area. The longer the insulator is, the smaller the heatsink area will be and this causes the spark plug to be hotter.
The heat range of the plug is defined as, its ability to dissipate heat created during combustion. If too cold a plug is used, the possibility of fouling is increased. A plug that is too hot can cause preignition. Hotter plugs tend to burn cleaner with less deposit build-up. They can be used for stop-start city driving, but not for extended high speed or load conditions as engine damage can occur.