Let's say you take a coil of wire perhaps 6 feet (2 meters) in diameter, containing five or six loops of wire. You cut some grooves in a road and place the coil in the grooves. You attach an inductance meter to the coil and see what the inductance of the coil is.
Now you park a car over the coil and check the inductance again. The inductance will be much larger because of the large steel object positioned in the loop's magnetic field. The car parked over the coil is acting like the core of the inductor, and its presence changes the inductance of the coil. Most traffic light sensors use the loop in this way. The sensor constantly tests the inductance of the loop in the road, and when the inductance rises it knows there is a car waiting!
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