The temperature in a gas oven is controlled by the expansion of an invar rod. Invar is a nickel – steel alloy (36% nickel) and has an extremely low coefficient of thermal expansion. It expands by only one millionth of its length per ° K rise in temperature.
The flow of gas to the oven burners passes through a valve A having an invar rod. The rod is attached to the closed end of a brass tube projecting into the top of the oven.
When the burners are lit the oven begins to warm up and the brass tube expands. The expansion of the invar rod is small enough to be ignored, so the end of the invar rod attached to the closed end of the brass tube moves to the left, reducing the size of the opening and reducing the gas flow. If the temperature of the oven fall, the brass tube contracts and the invar right moves to the right, increasing the gas supply. The thermostat is provided with a rotating knob to vary the msize of the valve opening and the steady temperature in the oven.