A gas expansion thermometer is made of a cylindrical bulb filled with inert gas & uses the volume expansion of gases proportional to temperature changes. Gas pressure changes inside the bulb are sensed by a special helical coil bourdon tube which is connected to an amplifying device that will give the pointer movement proportional to the temperature. The physical properties used will enable linear readings on the dial from the origin to full scale. When the reading is remote from the sensing point, a capillary is then used for transmission between the bulb and the thermometer head.