The working of the hand-held metal detector most widely used by security personnel for human frisking is based on the principle of
change in reluctance of an iron core in presence of a metallic object
change in conductance of an iron core in presence of a metallic object
electric field induced by a metallic object
eddy current generation in a metallic object
ANSWER
The detector works on eddy current generation in a metallic object, option D.
Official answer: DSource: Official answer key, GATE 2025 IN (IIT Roorkee)
A hand-held frisking detector has a transmitter coil carrying an alternating or pulsed current, which produces a time-varying magnetic field.
When a metallic object enters this field, the changing flux induces circulating eddy currents in the metal (Faraday's law, ).
The eddy currents set up their own magnetic field, which opposes the original one (Lenz's law). This changes the coil's inductance and losses, or the signal in a receiver coil.
The electronics detect this change in coil impedance, oscillator frequency or received signal, and sound the alarm.
Options A and B need an iron core whose reluctance or conductance changes. A hand-held frisking unit has no such core, and a non-ferrous metal such as aluminium or brass would not change a core's reluctance anyway, so these are wrong.
Option C is wrong because the detector does not sense an electric field produced by the object. It works through a magnetic field that the object reacts to.
Hence the principle is eddy current generation in the metallic object: option D.
KEY IDEA
A coil's alternating magnetic field induces eddy currents in any conductor that comes near. Those currents produce an opposing field, which changes the coil's impedance or the received signal, and this change is what the detector senses.
WATCH OUT FOR
Picking a variable-reluctance answer (A or B) because it sounds like an inductive sensor. Reluctance change needs a ferromagnetic core in the sensor and responds to ferrous material. Metal detectors work on eddy currents, which form in any conductor.
From the GATE 2025 IN paper. Every question of this paper · Download the full paper · All 19 Sensing principles questions, with notes
