A thermometer is a device for measuring temperature or temperature gradient. There are two important elements in a thermometer: a temperature sensor (such as a bulb in a mercury in glass thermometer or a high temperature sensor in an infrared thermometer), in which some temperature changes. (2) Some methods of converting this change to a numerical value (for example, a visible scale on a mercury in glass thermometer or a digital reading on an infrared model). Thermometers are widely used in technology and industry to monitor processes, meteorology, medicine and scientific research.

Some principles of thermometers are familiar to Greek philosophers two thousand years ago. Modern thermometers evolved from thermometers. At the beginning of the 17th century, the scale of thermometers was increased, and standardization was realized in the 17th and 18th centuries.

Although a single thermometer can measure the degree of high temperature, it is not possible to compare the readings on two thermometers unless they meet the agreed scale. There is an absolute thermodynamic temperature scale today. Internationally recognized temperature scales are designed to approximate this based on fixed points and interpolated thermometers. The latest official temperature scale is the 1990 international temperature scale. It extends from 0.65 K (− 272.5 ° C; − 458.5 ° f) to approximately 1358 K (1085 ° C; 1985 ° f).

Old thermometers are unregistered. That is to say, the thermometer does not keep the temperature reading after moving to different places. To determine the temperature of a pot of hot liquid requires the user to leave the thermometer in the hot liquid until the reading is finished. If the misaligned thermometer is removed from the hot liquid, the temperature indicated on the thermometer will immediately begin to change to reflect the temperature of its new condition (in this case, air temperature). The purpose of recording the thermometer is to maintain the temperature indefinitely so that it can be taken out and read out in a later or more convenient place. Mechanical alignment thermometers maintain the recorded maximum or minimum temperature until manually reset (for example, by shaking the mercury in glass thermometer) or experience more extreme temperatures. Electronic recording thermometers can be designed to remember the maximum or minimum temperature, or any temperature that occurs at a given point in time.

Thermometers are increasingly using electronic means to provide digital displays or inputs to computers.

Thermometers use a series of physical effects to measure temperature. Temperature sensors are used in various scientific and engineering applications, especially in measurement systems. The temperature system is mainly electrical or mechanical, sometimes it is inseparable from the system they control (such as mercury in glass thermometer). Thermometers are used in cold weather roads to help determine if icing conditions exist. Indoors, thermistors are used in climate control systems, such as air conditioning, freezers, heaters, refrigerators, and water heaters. Because of the limited measuring range of Galileo thermometers, they are used to measure indoor air temperature.

The liquid crystal thermometer (using thermochromic liquid crystal) is also used in the emotional ring and is used to measure the water temperature in the fish tank.

FBG temperature sensors are used in nuclear power plants to monitor the core temperature and avoid the possibility of nuclear meltdown.