This simple system uses a thermopile to generate all the power that is needed to operate the gas valve. The thermopile puts out millivolts. This type gas valve is used for floor furnaces,deep fryers, roofing equipment, etc. A fuse is tied into the jumper wire to safety magnet to kill the equipment should the thermostat for some reason not shut down at the desired temp. The most common problem with this wonderful system is the thermophile gets weak. When it gets weak it will hold the safety magnet feature but it will not operate the gas valve solenoid and when the solenoid is actuated the whole thing dies.Solution: replace the thermophile. The other problem might be the limit or the control thermostat has failed. In some instances a high limit melting type fuse will kill everything if it gets too dang hot.
This thermophile/pilot assembly when lit powers the valve. All that is needed to regulate the heat is a cheap wall thermostat