Temperature is measured by an instrument called a thermometer.
Thus a small amount of expansion or contraction of the mercury in the bulb, caused by heating or cooling, produces a noticeable rise or fall in its level in the tube. The tube's thick, curved face acts as a magnifier that makes the thin thread of mercury clearly visible. In making a liquid thermometer, the mercury is ordinarily driven to the top of the tube by heating. The glass is then sealed off, resulting in a vacuum when the mercury contracts during cooling. For high-temperature applications the tube is filled with a pressurized gas before sealing to prevent the mercury from boiling.