Page "Magnetic anomaly detector" Paragraph 10

During the Vietnam War the original AC-130A Spectre gunships and the A-6C Intruder employed the " Pave Mace "/" Black Crow " magnetic anomaly detection system to detect truck ignition coils in vehicles hidden under heavy jungle canopies.

When the electric current flowing through such a coil changes it produces a magnetic flux, which is, in simplest terms, a change in the magnetic field over a given area.

In particular, when the trucks were turned on the current flowing through the coils went from zero to some maximum, producing a large and easily detectable flux.