Which type of heat detector automatically resets after operation, when the ambient temperature drops below...

A rate-of-rise heat detector is a type of heat detector that automatically resets after operation, when the ambient temperature drops below the operating temperature. This is because a rate-of-rise heat detector operates on a rapid rise in element temperature of 6.7° to 8.3°C (12° to 15°F) increase per minute, irrespective of the starting temperature1 If the temperature rise stops or reverses, the detector will reset itself and stop sending an alarm signal to the control panel2 A rate-of-rise heat detector can also have a fixed temperature element that will trigger an alarm at a specified temperature, but this element is not self-resetting and requires replacement after activation2

References:

Heat Detectors Selection Guide: Types, Features, Applications - GlobalSpec2

Heat detector - Wikipedia1