Which device is composed of two or more coils linked by magnetic lines of force used to transfer energy from one circuit to another?

• A. Transformer
• B. Synchro System
• C. Solenoid
• D. Tachometer

Answer: (A)

The concept here is energy transfer between circuits through magnetic coupling using more than one coil. That matches a transformer. When you energize the primary coil with alternating current, it creates a changing magnetic field that links to the secondary coil through the magnetic core. This changing flux induces a voltage in the secondary coil, allowing power to move from one circuit to another without a direct electrical connection. The turns ratio between the coils sets whether the output voltage is stepped up or down, and the core helps concentrate the flux to keep the coupling efficient.

The other devices don’t fit this role: a solenoid is a single coil used mainly to produce a magnetic field for actuation, not to transfer energy between circuits; a tachometer measures rotational speed; a synchro system is used for position signaling or synchronization, not general energy transfer between circuits via mutual inductance.