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 from the time of Galileo 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.

Laboratory thermometers 
A laboratory thermometer is a tool used in laboratories, places where scientists and science techs perform experiments and measure things.  A laboratory thermometer can measure temperature very closely.  A laboratory thermometer can be put into the liquid or other thing that the scientist wants to measure.  A laboratory thermometer has a long stem with a silver bulb at the end. The silver color in the bulb usually means there is mercury in it.  Mercury becomes bigger as the temperature becomes hotter.  But mercury is a poison to humans.  Mercury-in-glass thermometers are less used in the 21st century because people want digital, alcohol-filled, and organic-based thermometers instead.

Medical thermometers 

In the 20th century, the traditional clinical thermometer was a mercury-in-glass thermometer. People put the end of this in their mouth (oral temperature), under their arm, or in their rectum (rectal temperature).

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 still 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 dangerous if it leaks out. 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 tympanic membrane (the eardrum) with infrared; band thermometers test a person's temperature on the front of their head.

Kinds of thermometers 
 Liquid-in-glass thermometer
 Mercury-in-glass thermometer
 Alcohol-in-glass thermometer
 Clinical thermometer
 Digital thermometer
 Rotary thermometer
 Resistance thermometer
 Liquid crystal thermometer
 Infra-red thermometer

References 

Laboratory equipment
Measuring tools
Temperature
Weather instruments
Thermometers