Thermometer
A bulb thermometer consists of a sealed, narrow glass tube, called a capillary tube. It has a glass bulb at one end and is filled with colored alcohol or mercury. Because mercury is poisonous, mercury thermometers are rarely used in medicine. The thermometer works on the principle that the volume of a liquid changes when the temperature changes. When warmed, the liquid in the bulb takes up more space and moves up the capillary tube. Interpreting Diagrams Why is a thermometer with a narrow tube easier to read than one with a wide tube?
Measuring temperature Thermometers are useful scientific instruments. They can be used to measure the static temperature of a material, or to record the change in temperature of a substance being heated, as shown above.