This is the most common type of heat detector. Fixed temperature detectors operate when the heat sensitive eutectic alloy reaches the eutectic point changing state from a solid to a liquid.
Thermal lag delays the accumulation of heat at the sensitive element so that a fixed-temperature device will reach its operating temperature sometime after the surrounding air temperature exceeds that temperature.
The most common fixed temperature point for electrically connected heat detectors is 58°C (136.4°F). Technological developments have enabled the perfection of detectors that activate at a temperature of 47°C (117°F), increasing the available reaction time and margin of safety.