The gas thermostat
Oven temperature control in a gas-cooker is effected by utilizing the exceedingly low thermal expansion of invar, an alloy of steel with 36 per cent of nickel. From the table on page 163 it will be seen that invar expands by only one-millionth of its length per kelvin rise in temperature.
The flow of gas to the oven burners passes through a valve A having an invar stem. This stem is attached to the closed end of a brass tube projecting into the top of the oven (Fig. 15.10). When the burners are lit the oven begins to warm up and the
brass tube expands. The expansion of the invar is negligible, and so it moves to the left, partially closing the valve opening and reducing the gas flow. Should the temperature of the oven fall, the brass tube contracts and the invar rod moves to the right, thus increasing the gas supply. The thermostat is provided with a rotating knob, not shown in the diagram, which varies the minimum opening of the valve and so controls the steady temperature to be maintained.