What is a resistance temperature detector, commonly known as an “RTD”? An RTD is a temperature sensor which, when subjected to a change in its body temperature, exhibits a positive, predictable and nearly linear change in electrical resistance. The origin of platinum RTDs is traced back as early as the late nineteenth century when C.W. Siemens wrapped platinum wire around a clay mandrel and inserted the device into an iron sheath. Though this may have been the first known “resistance temperature detector”, it was much too unstable and delicate to be used for precision thermometry. It would be some 60 years later before the RTD design was improved upon enough to provide precision temperature measurement capabilities. Today platinum RTDs provide reliable and precision temperature measurement for critical applications worldwide.