A gas thermometer measures temperature by the variation in volume or pressure of a gas.[1] One common apparatus is a constant volume thermometer. It consists of a bulb connected by a capillary tube to a manometer. The bulb is filled with a gas such that the volume of the gas in the bulb remains constant. The volume is related to temperature by k, known as Charles's Law . The pressure of the gas in the bulb can be obtained by measuring the level difference in the two arms of the manometer.[2] Gas thermometers are often used to calibrate other thermometers.[3]