What is the main function of a relay?

The main function of a relay is to open or close a circuit electrically using a low-power signal. A relay is an electromechanical switch that utilizes an electromagnet to operate a switching mechanism. When a low-power signal is applied to the relay's coil, it creates a magnetic field that causes the switch to either connect or disconnect contacts in the circuit it controls. This allows a small control signal to manage a larger load, enabling operations in various applications without requiring a direct interface with the high-power circuit.

The other options address different functions: converting AC to DC is typically the role of a rectifier, storing electrical energy is done by capacitors or batteries, and measuring voltage involves devices such as voltmeters. Therefore, the unique characteristic of a relay lies in its ability to control larger currents and voltages with minimal input, making it a crucial component in automation and control systems.