What device controls one electrical circuit by opening and closing contacts in another circuit?

The device that controls one electrical circuit by opening and closing contacts in another circuit is a relay. A relay is an electromechanical switch that uses an electromagnetic coil to operate the switch mechanism. When a current flows through the coil, it generates a magnetic field that pulls a lever or arm, either closing or opening the contacts of the relay, which in turn controls the flow of current in the circuit connected to the contacts. This allows a low-power signal to control a higher power circuit, making relays essential in various applications, such as in automation systems, safety devices, and in the control of motors.

In contrast, a capacitor stores and releases electrical energy but does not perform switching functions. A transformer is used to change voltage levels in an alternating current circuit; it does not open or close contacts. A resistor limits the flow of current in a circuit and does not switch or control one circuit through another. Thus, when looking for a device specifically meant for switching control in electrical circuits, the relay is the clear choice.