Temperature measuring instruments can be classified into two main categories: contact and non-contact.

Contact thermometers physically touch the object or substance they are measuring the temperature of.

• Liquid-in-glass thermometers: These are the classic thermometers you might see in a doctor's office or laboratory. They contain a liquid, typically mercury or alcohol, that expands and contracts with changes in temperature. The scale on the side of the thermometer indicates the temperature.
• Thermocouples:These devices consist of two dissimilar metals joined at one end. When the junction is heated, a voltage is created that is proportional to the temperature. Thermocouples are widely used in industry because they can withstand high temperatures and are relatively inexpensive.
• Resistance temperature devices (RTDs):These instruments measure temperature by detecting the change in electrical resistance of a metal wire or film as it is heated. RTDs are known for their accuracy and stability, but they are more expensive than thermocouples.
• Thermistors:Similar to RTDs, thermistors are semiconductors whose electrical resistance changes with temperature. However, thermistors have a much higher sensitivity than RTDs, making them suitable for measuring very small temperature changes.
• Bimetallic strips:These devices consist of two strips of metal with different coefficients of thermal expansion bonded together. When heated, the bimetallic strip will bend due to the unequal expansion of the two metals. Bimetallic strips are often used in thermostats to regulate temperature.

Non-contact thermometers do not physically touch the object or substance they are measuring the temperature of.

• Infrared thermometers:These instruments measure temperature by detecting the infrared radiation emitted by an object. The intensity of the radiation is proportional to the temperature of the object. Infrared thermometers are useful for measuring the temperature of moving objects or objects that are difficult to reach.
• Pyrometers:These are specialized non-contact thermometers that use the intensity of visible light emitted by an object to determine its temperature. Pyrometers are often used for measuring very high temperatures, such as those found in furnaces.