Answer:
I am assuming that you mean switching a millivolt gas value. The switch applies a voltage to the gas valve thus allowing gas to flow. The voltage is provided by a thermopile. This device transforms the heat from the pilot into a voltage that can be used to control the valve. This system does not require an external voltage source. It does require the pilot to continuously operate. A thermopile consists of a number of thermocouples that are wired in series. This allows an increase in the voltage that would be provided by a single thermocouple.