Thermocouples are made by joining two wires of dissimilar metals. The point of contact between the wires generates a voltage that is approximately proportional to temperature. Characteristics include wide temperature range (up to +1800°C), low cost (depending on package), very low output voltage (about 40 µV per °C for a K type), and reasonable linearity. There are several thermocouple types, which are designated by letters. The most popular is the K type.