In a carbon pile regulator, how are voltage fluctuations sensed?

• A. current coil
• B. voltage coil
• C. magnetic core
• D. voltage meter

Answer: (B)

The voltage fluctuations are sensed by a voltage coil. In a carbon pile regulator, the output voltage is monitored by a coil connected to the regulated circuit; its magnetic field is proportional to the sensed voltage. This field acts on a movable element that adjusts the pressure on the carbon blocks, changing the pile’s resistance to hold the output voltage steady. If the output climbs, the stronger magnetic pull reduces the carbon contact area (raising resistance), bringing voltage back down; if the output falls, the pull weakens and the contact area increases (lowering resistance) to raise voltage. A current coil would respond to current rather than voltage, a magnetic core alone doesn’t sense anything, and a voltage meter is just a measuring device, not part of the feedback loop.