A thermometer is a device that measures temperature. Thermometers use temperature sensor to measure a change in temperature, and convert that information in a voltage value that corresponds to temperature readings on a display or dial.
Thermistors and thermocouples are the most common temperature sensors. They are both inexpensive, widely available, and can measure a wide range of temperatures. By design, both sensors can readily convert a temperature change into an electrical signal: thermistors by measuring temperature-dependent resistance, and thermocouples by directly producing a temperature-dependent voltage.
Thermometers are ubiquitous in industrial applications, used to monitor ambient, process and equipment temperatures for climate control, safety and reliability purposes. Infrared thermometers measure thermal radiation, and can monitor temperature for similar applications from a distance.