What component sends a signal to several parts to activate a certain function in the system?

The component that sends a signal to several parts to activate a certain function in the system is typically an electromagnetic relay. An electromagnetic relay acts as an electrically operated switch. When an electrical current passes through the coil of the relay, it generates a magnetic field that moves an armature, closing or opening contacts. This action allows it to control multiple circuits and can activate various devices, making it possible to send a signal to different parts of the system simultaneously.

The other options serve specific functions but do not operate in the same way. A pressure switch responds to changes in pressure and can initiate actions based on pre-defined pressure thresholds but does not activate multiple functions at once. A temperature sensor detects temperature changes and typically provides readings rather than sending activation signals, while mercury switches are more limited in function and used primarily for detecting position or orientation rather than signaling multiple components for activation.