What device connects to a circuit and responds to a physical stimulus?

A device that connects to a circuit and responds to a physical stimulus is a detector. Detectors are designed to sense changes in their environment, such as heat, smoke, or gas, and then send a signal to the fire alarm system to alert occupants or trigger a response. Depending on the type of detector, it may respond to various physical stimuli, such as thermal energy from heat sources or particulates from combustion.

Unlike switches, which primarily act as on/off mechanisms without responding to environmental changes in the same way, detectors specifically monitor for safety-related signals. Detonators, commonly associated with explosive devices rather than fire safety systems, are not designed for this type of response. Transmitters, on the other hand, are typically used to send signals over distances but do not directly respond to physical stimuli in the way that detectors do. Thus, the role of a detector is crucial in fire alarm systems, as it directly correlates to the system's ability to sense and respond to potential fire hazards.