What is a relay?

A relay is fundamentally an electrically-controlled switch. Its primary function is to allow a low-power signal to control a higher power circuit. The relay typically consists of an electromagnetic coil and a set of contacts. When an electric current flows through the coil, it generates a magnetic field that moves an armature, either opening or closing the contacts to allow or interrupt the flow of current in the controlled circuit.

This ability to control high voltage and current levels with a small control signal makes relays particularly useful in various applications, such as automation, switching power levels, and isolating different circuit sections.

The other options describe different electronic components that serve separate purposes. A current controlled amplifier is designed to amplify small changes in input current, while an inverting amplifier primarily inverts the phase of the input signal. A pass transistor is used for regulating voltage in power supplies rather than acting as a switch. The function of each of these components differs significantly from that of a relay, which is specifically designed to switch circuits on and off based on electrical control signals.