What type of fire warning system is activated by the rate of temperature rise?

The correct answer is based on the principle of how a thermocouple operates in relation to temperature changes. A thermocouple fire warning system is designed to detect rapid increases in temperature, which are indicative of a fire. It consists of two different types of metal that produce a voltage when heated, allowing it to measure temperature differentials accurately.

When the rate of temperature rise exceeds a certain threshold, the thermocouple generates a specific signal, which can trigger an alarm to alert personnel about potential fire hazards. This makes it particularly effective in providing early warnings against fires, as it can react to sudden temperature spikes that often accompany combustion events.

Other options like photoelectric, ionic, and infrared fire warning systems function on different principles. Photoelectric systems detect smoke rather than temperature, ionic systems use ionization to identify particles in the air caused by smoke, and infrared systems focus on detecting heat signatures rather than the rate of temperature increase specifically. Thus, the thermocouple is uniquely suited for this role among the listed options.