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