In the field, volcanologists can use a thermocouple to measure the temperature of a steaming fissure or of a lava flow. A thermocouple is a device consisting of two different metallic conductors that are connected at both ends, producing a loop in which heat is converted into electrical current when there is a difference in temperature between their two junctions. It is used to measure the temperature of a third substance by connecting it to both junctions and measuring the voltage generated between them. The temperature is read off by a hand-held device.