<Li> Fixed contact screw . This is adjusted by the manufacturer . It is connected electrically by a second wire of the pair to the thermocouple and the heater's electrically operated gas valve . </Li> <P> Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat . </P> <P> As illustrated in the use of the thermostat above, all of the power for the control system is provided by a thermopile which is a combination of many stacked thermocouples, heated by the pilot light . The thermopile produces sufficient electrical power to drive a low - power gas valve, which under control of one or more thermostat switches, in turn controls the input of fuel to the burner . </P> <P> This type of device is generally considered obsolete as pilot lights can waste a surprising amount of gas (in the same way a dripping faucet can waste a large amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters and gas fireplaces . Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot still represents a direct heat gain for the water tank . The Millivolt system also makes it unnecessary for a special electrical circuit to be run to the water heater or furnace; these systems are often completely self - sufficient and can run without any external electrical power supply . For tankless "on demand" water heaters, pilot ignition is preferable because it is faster than hot - surface ignition and more reliable than spark ignition . </P>

Describe how a main gas valve gets electrical power to open
find me the text answering this question