Electrodes for gas boilers

Electrodes are used in boilers, gas fire and other gas appliance.
There are two type of electrodes - ignition electrode also known as a spark electrode and flame sensing electrodes also known as an ionisation electrode
The ignition electrode is used for creating a spark when a high voltage is applied to it. This the ignites the burner.
The flame sensing electrode is used to detect the presence of a flame. If a flame is present a small current is sent to the pcb which then tells the boiler what to do next.

Some appliances have just a spark electrode whilst others have both. In some appliance the same electrode is used for sparking and flame moitoring.