A thermometer is an instrument for measuring or showing temperature (how hot or cold something is). One type of thermometer is a narrow, concealed glass tube containing mercury or alcohol which extends along the tube as it expands. Another type is a digital thermometer, which uses electronics to measure temperature.
Early thermometers measured the expansion and contraction of air. After the middle 17th century many used alcohol or mercury. In the 19th century a mechanical thermometer was in invented that used a bimetallic strip to move a pointer. This kind is still popular where people like to read temperature from a distance.
It is only possible to find oral temperatures on patients who can hold the thermometer correctly in their mouth. So small children cannot use this method. It is also a problem for people with a cough or people who are vomiting. In the past it was a big problem because mercury thermometers needed a long time to measure the temperature. Today's digital thermometers are faster. If a person drinks something hot or cold, one needs to wait before testing their oral temperature.
When measuring a person's rectal temperature, it helps to use a cream on the thermometer. Rectal thermometers are usually more reliable since they aren't as much influenced by other factors. In some countries people think it is embarrassing to use them for people older than two or three. In other countries, it is considered normal for children and adults to use rectal thermometers.
In the 1990s, people in many countries thought mercury thermometers were too risky, as mercury is harmful to humans. Today we use electronic thermometers. Sometimes thermometers with liquids are used, but not with mercury.
There are other kinds of medical thermometers: tympanic thermometers test the temperature of the tympan (the eardrum) with infrared; band thermometers test a person's temperature on the front of their head.
Sorts of thermometers we often use
- Liquid-in-glass thermometer
- Mercury-in-glass thermometer
- Alcohol-in-glass thermometer
- Clinical thermometer
- Digital thermometer
- Rotary thermometer
- Resistance thermometer
- Thermistor thermometer
- Liquid crystal thermometer
- Infra-red thermometer
Images for kids
A clinical mercury-in-glass thermometer
Thermometer Facts for Kids. Kiddle Encyclopedia.