What is a device that uses a solenoid to operate a switch in another circuit called?

A relay is a device that uses a solenoid to operate a switch in another circuit. When an electrical current passes through the solenoid, it creates a magnetic field that can move a switch, thereby opening or closing a separate circuit. This allows low-power signals to control larger currents, making relays invaluable in many applications, such as automating processes or protecting circuits.

In contrast, a transformer is designed to alter the voltage of alternating current and does not use a solenoid to operate a switch. A capacitor stores electrical energy and releases it when needed; it doesn't function as a switch or control device. A diode allows current to flow in one direction only and doesn’t operate switches or circuits like a relay does. Thus, the defining characteristic of a relay is its ability to control a circuit through magnetic actuation, clearly distinguishing it from the other devices listed.