An RTD consists of a wire, typically made of copper, platinum, nickel, or tungsten, connected to a constant current source. A voltmeter is connected in parallel with the RTD to monitor the voltage drop across it. As the temperature of the RTD changes, its resistance changes, which causes a change in the voltage drop. The voltage measured across the RTD is scaled in degrees Fahrenheit or Celsius.
RTDs are used in applications where accuracy and reliability are critical.