What is a primary requirement for a thermocouple fire warning system to be activated?

A primary requirement for a thermocouple fire warning system to be activated is a rapid rise in temperature. This characteristic is crucial because fire situations typically involve quick increases in temperature due to combustion. Thermocouples are designed to measure temperature changes; therefore, they are particularly responsive to swift increases.

When the temperature rises rapidly, it serves as an indicator that a fire may be developing, prompting the system to trigger an alert. This prompt action allows for immediate measures to be taken to avoid potential danger.

The other options do not effectively facilitate the purpose of a thermocouple fire warning system. A slow rise in temperature may not indicate a fire hazard, as it could represent a benign condition rather than a heating event associated with combustion. An emergency power source, while important in many systems for reliability, does not directly relate to the sensor's ability to detect temperature changes. External temperature sensors can be part of a broader setup, but they don't specifically represent the fundamental requirement of temperature change measurement for triggering alarms in fire situations. Hence, the focus on detecting rapid temperature rises as a critical element of fire detection systems is what makes this option the correct choice.