A glass thermometer consists of a stem and bulb. The stem (long part) of the thermometer has a hollow shaft running almost the entire length of the stem. The bulb of the thermometer contains a small amount of mercury, a metal that is liquid at ordinary temperatures.

The thermometer is designed so that mercury from the bulb can enter the hollow shaft in the stem. If you hold the mercury thermometer so that the bulb is pointed up; however, the mercury will not flow into the shaft. The mercury only enters the shaft if the mercury in the bulb expands and some of the mercury is forced into the shaft.

The mercury is cooler than body temperature. When the thermometer bulb is placed next to body tissue, the mercury absorbs some of the body’s heat.

As the mercury gets warmer, it expands. Since the mercury has no more room in the bulb, some of the mercury is forced into the shaft. More and more mercury is forced into the shaft until the mercury reaches the same temperature as the body tissue and stops expanding. The patient’s temperature is determined by measuring how much the mercury expanded.