What device is used to detect abnormally high temperatures or a rapid rate of temperature increase?

The device used to detect abnormally high temperatures or a rapid rate of temperature increase is the heat detector. Heat detectors are specifically designed to respond to changes in heat levels and are particularly useful in environments where smoke detectors might produce false alarms or be less effective. They can be configured to respond to fixed temperature thresholds or to the rate of temperature rise, allowing for an effective response to fire conditions before a significant amount of smoke is present.

Smoke detectors, on the other hand, primarily detect particulate matter from combustion, which typically occurs after there has already been substantial heat and smoke production. Flame detectors work by detecting the visible light emitted from flames and are thus effective for fires that reach an open flame but may not respond in the early stages of a fire when heat is the primary concern. Carbon monoxide detectors detect the presence of carbon monoxide gas, a byproduct of incomplete combustion, and are not focused on temperature detection.

Given the specific function of the heat detector in assessing temperature changes, it is the most suitable answer to this question.