What is the primary function of an "automatic rescue device" in elevators?

The primary function of an "automatic rescue device" in elevators is to safely lower an elevator car to a designated floor during a power outage. This device is crucial for ensuring passenger safety in the event of an emergency where the elevator becomes inoperable due to a loss of electrical power.

In such situations, the automatic rescue device takes over the elevator's control systems and allows it to descend slowly and safely to a predetermined floor, typically the nearest appropriate landing. This feature is vital for preventing passengers from being stranded in the elevator and ensures they can exit the elevator without undue risk.

The other options, while related to safety and functionality, do not accurately describe the primary role of the automatic rescue device. Alerting passengers during an emergency, providing additional ventilation, and monitoring weight capacity are important aspects of elevator safety and operation but are not the core function of the automatic rescue device.