I believe that a capillary was described earlier.
Capillary tubes work off of the pressure of heated gas. The most common uses in a rv or home is to control the temprature in ovens and in some cases refrigerators. They are not normally replaceable.
Termocouples produce small amounts of electricity through bi-metal when heat is applied correctly. They produce enough electricity to hold open a pilot valve. Thermopiles are a group of thermcouples packed into a case to produce enough electricity to control a furnace
gas valve through a thermostat.
I try to have about 1/2" of the top in the flame.