The mercury flame sensor is a mechanical device that proves the existence of an acceptable pilot flame. It converts the heat of a pilot flame to motion which is used to open and close a set of electrical contacts. When the bulb is heated by a pilot flame, the mercury is vaporized causing pressure in the capillary and diaphragm. Movement of the diaphragm causes the snap-switch to open one set of contacts and close a second set. These contacts control the pilot valve and the main valve.