Which component of a fire detection system is crucial for circuit completion?

The thermal switch is vital for circuit completion in a fire detection system because it is designed to respond to temperature changes associated with a fire. When the temperature reaches a specific threshold, the thermal switch closes the circuit, allowing the electrical current to flow. This action can trigger alarms, indicator lights, or other responses like automatic firefighting systems.

The indicator light serves as a visual signal but does not play a direct role in completing the circuit. Similarly, the alarm bell indicates that a fire has been detected but is also not part of the circuit completion process. The power source is essential for providing energy to the system but does not directly engage in the sensing of fire conditions or triggering the alarm response. Thus, the thermal switch is the correct choice for its critical role in detecting heat and completing the circuit when fire conditions are met.