ARRL Technician Practice Exam

What best describes a relay?

1. A switch controlled by an electromagnet

2. A type of fuse

3. A circuit breaker

4. A variable resistor

The correct answer is: A switch controlled by an electromagnet

A relay is best described as a switch controlled by an electromagnet. This device functions by using an electromagnetic coil to create a magnetic field that enables the opening or closing of a switch. When an electrical current passes through the coil, it activates the electromagnet, which then physically moves the switch contacts, allowing or interrupting the flow of electricity through another circuit. This mechanism allows relays to control high-voltage or high-current circuits using a low-voltage signal, making them essential components in various electrical systems, including automotive and industrial applications, as well as in home automation. The operational principle distinguishes relays from other options, which do not share this electromechanical switching function. For instance, a fuse protects circuits by melting under excessive current, while a circuit breaker interrupts current flow mechanically but does not operate using an electromagnet. A variable resistor, on the other hand, alters resistance within a circuit rather than serving as a switch. Thus, the unique operation of a relay as a switch driven by an electromagnetic force is what makes this description the most accurate.