The interrmittent pilot

When gas appliance manufacturers began looking for ways to increase efficiency, one of the first methods was to eliminate the standing pilot.  The standing pilot was using gas all the time.  Regardless of whether the appliance was heating or not.  The gas usage of the pilot is a waste of fuel.  The video below shows how much gas the pilot uses.  

To eliminate the pilot usage, we need to devise a control that will light the pilot upon a call for heat, prove its on so the burner will not come on if the pilot is not burning and shut it off when the call for heat stops.  There are several methods to accomplish this.
One of the first methods was the 3 wire pilot assembly.  This system was used by Carrier corp products. ​

The pilot assembly above is an example of this part.  On top is the spark probe.  When the spark lights the pilot, the flame heats a bimetal on the opposite side of the pilot.  The bimetal moves and changes the position of the switch connected to the wires above.