Which device is composed of two or more coils linked by magnetic lines of force used to transfer energy between circuits?

• A. Servo
• B. Thermistor
• C. Synchro System
• D. Transformer

Answer: (D)

Energy transfer between circuits through magnetic coupling using two or more coils is what a transformer does. When alternating current flows in the primary winding, it creates a changing magnetic flux in the core. That flux links the secondary winding and induces a voltage there. The voltage ratio follows the turns ratio of the windings, Vp/Vs = Np/Ns, and the currents adjust inversely so that power is conserved (minus losses). This lets you step voltage up or down, provide isolation, or match impedances between circuits.

Other devices listed don’t fit this description. A servo is an actuator used for precise motion control, not just energy transfer between circuits. A thermistor is a temperature-dependent resistor, with no magnetic coupling. A synchro system uses electromagnetic coupling for position indication, but its primary purpose is sensing angle rather than transferring energy between circuits.