A fire-warning device designed to be bolted to a flat surface that may rise to a dangerous temperature. It consists of a cavity filled with 20 cm ₃of ethyl or butyl mercaptan alone or with other stench agents and is sealed with a fusible plug in a brass container with a hexagonal head, arranged to liberate the stench agent at any temperature chosen. Tests in pits have shown that a strong smell could be detected 1.7 miles (2.7 km) from the discharge point 25 to 30 min after the device operated.