A standard thermometer is usually a bulb or a spring thermometer. Both work by having a liquid, either alcohol or mercury, in a closed space with a vacuum, where the liquid expands as the temperature rises. Colored alcohol or mercury rise along a scale in a bulb thermometer; expanding liquid rotates a spring to turn an indicator needle around along a circular scale on a spring thermometer. Thermometers now often have digital as opposed to scale displays.
Temperatures are sometimes measured by a thermocouple. Two metal leads of dissimilar metals are placed in close proximity to each other, creating voltage. Changes in voltage correspond to changes in temperature. Thermocouples are used in industry, and are often connected to other devices that turn mechanisms off and on in response to certain temperatures. Thermocouples are not as accurate as thermometers.
Thermocouples are increasingly being replaced by resistance temperature detectors, or resistance thermometers. RTDs use carbon or platinum sensors to detect changes in electrical resistance. These changes are caused by temperature changes, and the changes are predictable. RTDs exploit that predictability to determine temperature. RTDs are generally more stable and predictable than thermocouples. A consistent light current is passed through the RTD, past the leads, and then resistance can be determined and temperature calculated.
A pyrometer measures the surface temperatures of objects. It is a tool that combines an optic feature with a temperature reader constructed of an ultrathin filament. The pyrometer is aimed at the surface of an object, whereupon the optic device focuses on the thermal signature -- or radiated heat -- and transfers that heat to the filament reader. These are especially useful for measuring temperatures on surface that are out of reach or too hot to touch, like steam boilers, metallurgy furnaces and hot air balloons.
Irving Langmuir is a Nobel Prize winning physicist. Langmuir wanted to learn how to take the temperature of electrons as part of his research to learn about the electrical potential of plasma, a gas-like condition of matter in which some particles lose electrons. Langmuir invented a device called the Langmuir probe that does just that, by placing electrodes in the plasma, then measuring currents in the plasma. Langmuir probes are not in everyday use.
Non-visible light corresponds with heat. When you look at things, you see visible light. A red fire truck looks red whether the temperature is freezing or 100 degrees Fahrenheit. But if you look at things through an infrared detector, you can see "heat signatures," that is, changes in things based on temperature. By attaching a meter that measures infrared light frequency, the infrared thermometer can -- like the pyrometer -- take surface temperature measurements from a distance.