RTD’s are made with metal filaments of 1 of 3 different materials. Platinum, Nickel or copper are the 3 main materials they are made with. However platinum apparently has the best temprature to resistance range of these 3 materials. Pure platinum has a resistance change of 0.003925 ohms per degree C. It is the most stable and the widest temp range -272.5 C to +961.8 C. Platinum is also the material of choice for its chemical inertness. So when metal heats up resistance goes up and temp down and resistance down too.