Standard platinum resistance thermometers. Platinum resistance thermometers (PRTs) are used to record temperature as the resistance of a piece of platinum wire. PRTs are used for international standards relating to temperature measurements due to the stability and ductility of platinum, and the reproducibility of the results. The UK's National Physical Laboratory (NPL) defines such standards by calibrating PRTs such as these. At top is a PRT used in cryogenic settings. The other three PRTs are sheathed, with one sheathed in steel (centre) between two sheathed in glass (above and below). Photographed at the NPL, Teddington, UK.