Probably the oldest method of automatically igniting a main burner is with a pilot light.

Question

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Pilot lights have been used for many years to ignite the main burner in gas appliances such as ovens, water heaters and furnaces. The pilot light is a small flame that burns continuously and is used to ignite the gas when the burner is turned on. However, newer appliances often use electronic ignition systems instead of pilot lights.

Probably the Oldest Method of Automatically Igniting a Main Burner